Reddit mentions of BIKEHAND Bicycle Bike Chain Wear Indicator Tool Chain Checker - Compatible with Shimano Sram KMC and All Others 7 to 12 Speed Chains

I was gifted the Big Blue Book of Bicycle Repair at a bike shop I used to work at. Fantastic reference book, good descriptions, good photos. That and/or youtube are good places to learn maintenance.

Now, on to your bike...

>All the spokes are pretty much completely rusted

Stop there, new wheels are required. Determine if they're 700c or 27" the tire should tell you this info, if you can read it. On the subject of new wheels, I'd also recommend new tires/tubes as they've probably sat through the rusting phase.

>rear brake squeaks like crazy and you really have to pull it to get it to work

New brake pads. The current ones are most likely dried out, err on the side of caution here, brakes will literally save your life (Don't listen to Premium Rush on the topic).

>Rear tire slides against rear brake hindering movement slightly

Brake pad adjustment necessary. Will happen when replacing pads.

>Chain and rear derailer seem to be in good shape

Close up pictures required. You could probably just get a chain cleaner, hopefully the "black greasy stuff" protected the chain from corrosion. Rear and front derailleurs could probably benefit from a healthy application of triflow at the joints.

>Brake cables are rusty, definitely need to be replaced

Right on the money here. New cables and housings. Triflow the shifters for good measure. Replace if they don't work after new cables/housing.

>The gear changer on the left handle bar doesn't work

Triflow trick from above, let it soak for like an hour. Do you still hear "clicks" when you press it? Are the paddles stuck? If your answers are no and yes (respectively), replace anyway.

Lastly, if you take the chain off, how smoothly do the cranks spin? "Stuttering" cranks sometimes mean your bearings and/or races are pitted. New bottom bracket probably isn't a bad item to add to the list, the Shimano UN57 (or similar) are relatively inexpensive and will probably outlast the next apocalypse.

It probably can't hurt to put a wear indicator on the chain and cassette to ensure they are still good to go. This could cause for some harder to diagnose issues down the road for home mechanics if they do need replacing. Chain wear indicators are easy to use, cassette wear indicators require a little more finesse and practice.

If in doubt, grease. You can't go wrong with a little more grease on the moving parts on this bike.

There is a tool like this you can buy. When my chain is stretched to the .75 I replace it.

Here is a better tool: https://www.amazon.com/Park-Tool-Chain-Indicator-CC-3-2/dp/B000OZFIMQ

Good stuff! Consider purchasing a chain wear indicator so that you'll be able to see when it's time to replace the chain in the future.

Worn down chains are actually a larger issue than you might think because they cause uneven stress and wear upon you cassette and chainrings, which causes them to wear faster as well.

Good riding!

Go ahead and buy a new cassette and two chains. They are wear items, you will use them. If you wait too long replacing the chain, it will accelerate wear on the cassette and chainrings. I just bought a chain for $16, totally worth it to replace a couple hundred miles early to avoid chewing up a $40 cassette or having it break on my way to work.

I have this chain checker and it's pretty nice, and the price is great.

https://www.amazon.com/BIKEHAND-Bicycle-Chain-Indicator-checker/dp/B00811WQW0/ref=sr_1_2?ie=UTF8&qid=1499380143&sr=8-2&keywords=chain+stretch+gauge

Yes, you can use a ruler, but for $4 it makes the job easier and as long as you use this tool it will save hundreds.

https://youtu.be/Y54ekC8PHB0

You need a chain wear indicator. They can be had for very cheap ($5 on amazon):

https://www.amazon.com/dp/B00811WQW0/ref=cm_sw_r_cp_apa_O7LtzbG5NRZPR

Fancy. Could always opt for the old school tool that is a fraction of the cost.

http://www.amazon.com/gp/aw/d/B00811WQW0?cache=5485c8fa147a8c87b577215ce1895307&pi=SY200_QL40&qid=1404813284&sr=1-1#ref=mp_s_a_1_1

re: Sizing,
Everyone's different. Go to your local bike shop and try sitting on a medium and a large of a brand's line, then try those on another brand. You're generally on the cusp of a large, which as far as I know most people, in general, recommend going a size up if you're on that cusp. All depends on your reach, etc.

People below are saying to go with the Meta. I disagree for one major reason: dropper posts are present on the other two. This is a $200-300 upgrade that I would argue is the next biggest thing since rear suspension. Additionally, the drivetrains on the Giants are all 1x's which I would also prefer. You're also looked at tapered tubes for the Giants, which is the new industry standard for forks. The 2015 is locked into a somewhat old paradigm.

Nevermind crossed out text from above, looks like it has a routed dropper, but still appears to have a 2x drivetrain?

Go with either Fantom. I'd suggest going to try riding the large first, then try the medium to see if you feel 'restricted.'

Someone please argue all my points. :D

Edit : /u/letsplaypsvr made a very valuable point about buying new, I think one of the major things to look at is the chain wear. Buy a small tool for cheap that measures chain wear for 1x10-11 and take it with you to check out the bikes. If the chain's not been changed when it should've, it may have damaged the drive train to the point that a new chain would degrade hyperquickly and cause even more damage to the drive train.. this is yet again.. another $200ish in parts/time/tools. If you can identify that the drive train's shot and needs replacing, you could always haggle way down.

In addition to the other advice here, I would recommend picking up a chain wear checker (example). Use it every once in a while, and it'll tell you when you need to replace the chain, before the worn chain tears up the gears too badly.

One set of gears should last 3 or 4 chains if you keep your chains lubed and replace them when they wear out, but if you keep waiting until the chain starts skipping, you're likely to need new gears with every new chain.

Get a chain cheking tool. Chain wear can vary dramatically with seasons, and looking into it only every 6 months can mean an expensive drive-train replacement.

I'm not sure about checking the tension of the chain but there is stuff like this