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Depending on what you are looking to do to get back running you obviously need a new derailleur, you can pick up a 7 speed Shimano for relatively cheap (only Shimano, SRam/Campagnolo use different pull ratios and therefore wont work with your shifter) the exact one you have is in this link https://www.amazon.com/Shimano-8-Speed-Mountain-Bicycle-Derailleur/dp/B0064QKTW8

Looking at the shape of your derailleur and the amount of torque that thing saw there is a good chance you will need a new derailleur hanger from the looks of it you might be #13 http://derailleurhanger.com/specialized.htm Now, there is a chance the old hanger is fine, you could always throw the new derailleur on the bike first and see if everything is lined up (if the derailleur is not parallel with the gears you need a new hanger).

I would also look at putting in new cable and housing, you can pick that up pretty cheap, just make sure it's for shifting and not brakes.

When you look to hook all this stuff up do yourself a favor, watch a few youtube videos about adjusting your new derailleur, it's not that hard but a proper setup can make a huge difference, and it's best to have it dialed before you start skipping gears on your ride. New chain as you mention is a no brainier, make sure it's a 7 speed chain (other chains are different widths). When you get your new chain clean it with some simple green or other degreeser (while you have that degreeser out give that cassette a good cleaning) once you have it all good and clean rinse, rinse, rinse, don't leave any solvent on the chain or cassette, then use some chain lube (not WD40 or oil), depending on where you live you may want different lube (follow directions, some require quick wiping some don't).

Last piece of advice is take those brakes slow, those pads might be shot.

I think you're rolling a 2000 version of your bike the 2003 version was 8 speed but you can check the specs listed against what's on your bike and narrow it down.
http://www.bikepedia.com/QuickBike/BikeSpecs.aspx?Year=2000&Brand=Specialized&Model=Hardrock&Type=bike

Good luck, get rolling.

Suggestion #1 - find your local bike co-op, wheel your junk in there, let them help you.

Suggestion #2 - you'll get the best bang for your buck if you buy a wheel and hub already laced up, but you'll need to know what size the hub is. Post up your bike make/model/year, or just measure the space between the front dropouts. You'll also need to know if you're looking for a rim brake or disc brake setup. And then if it does have disc brakes, you'll need a brake disc. WTB Nanos are a pretty standard XC tire, the Nano 40 is probably the closest thing they sell to what you've got, but really anything will do. XC tires tend to have light weight casings and small knobs, closely spaced.

Assuming a 100mm front hub spacing and disc brakes, something like this would do you: https://www.amazon.com/Wheel-Master-Alloy-Mountain-Double/dp/B0186OUK3I - there's better out there for sure, but at the budget you're playing with they're all pretty much the same I think. You'd have to pay several hundred more for a "good" wheelset.

Oh, and don't forget the QR skewer or thru-axle, depending on which setup you have.

They make extractor kits for situations like this.

boop

Best of luck to you!

It’s called a derailleur hanger. They are replaceable. Either call GT for an OEM replacement or look online, https://www.amazon.com/Flex-Derailleur-Aggressor-Avalanche-Timberline/dp/B01LWC9Y8Y

If you want to buy from amazon here's an entry level bike that will stand up to just about anything you wanna throw at it!
Diamondback Bicycles Hook 27.5, Silver, 20" / Large https://www.amazon.com/dp/B01LWKEJYX/ref=cm_sw_r_cp_api_u3bmzbF61HWW3

If you need it to be "correct", you'll need more research. If you want it to function, I'd get these-

Shimano RevoShift SL-RS35 Friction 3x7 Speed Shift Lever Set Right/Left 21 Speed Twist Shifter https://www.amazon.com/dp/B01E867WDE/ref=cm_sw_r_cp_apa_oF-gybZC6TGGJ

You need this.
https://www.amazon.ca/Baleaf-Padded-Bicycle-Cycling-Underwear/dp/B00MTE6NEW/ref=sr_1_fkmr1_2?keywords=biking+shami&qid=1562944487&s=gateway&sr=8-2-fkmr1

https://www.amazon.com/FOOKER-Composite-Mountain-High-Strength-Bikesflat/dp/B07D66NDS3/ref=mp_s_a_1_1_sspa?keywords=mtb%2Bpedals&qid=1562824639&s=gateway&sr=8-1-spons&psc=1&th=1

There are some bluetooth speakers that can attach to different things, I have these, and they would probably work. I would only use them on very tame trails.
http://www.amazon.com/UE-Wireless-Bluetooth-Waterproof-Shockproof/dp/B00YARCGHO

https://www.amazon.ca/Shimano-MF-TZ21-14-28-Teeth-Freewheel/dp/B003EQ71S6/
Lots of under $25 options.

https://www.amazon.com/Mr-Tuffy-Bicycle-Purple-2-0-2-35/dp/B003RLK19Y/ref=sr_1_fkmr3_3?s=sporting-goods&ie=UTF8&qid=1493355335&sr=1-3-fkmr3&keywords=slime+tire+liner+29

edit: had to change because I realized you have 27.5"

https://www.amazon.com/gp/aw/d/B01LZOIKX9/ref=mp_s_a_1_2?ie=UTF8&qid=1509820272&sr=8-2&pi=AC_SX236_SY340_FMwebp_QL65&keywords=Bell+bike+computer&dpPl=1&dpID=51FK4z7FSIL&ref=plSrch

what about these? they come with grips too!

https://www.amazon.com/gp/product/B0017YX2OO/ref=ox_sc_act_title_1?ie=UTF8&psc=1&smid=ATVPDKIKX0DER

I was thinking about putting these on it. Do you think these will give me the smoothness I want on pavement and traction on dirt and trails?


https://www.amazon.com/Kenda-Pathfinder-Bicycle-Blackwall-1-95-Inch/dp/B002DX1DGW/ref=sr_1_2?ie=UTF8&qid=1478472498&sr=8-2&keywords=kenda+pathfinder