Reddit mentions of Silverline Tools - Metric Dial Indicator - 0 - 10mm

Aside from good calipers & feeler gauges.. You might want something like a dial indicator. You'll often see them used on lathes for testing roundness but they also work great for tramming your print bed. Print out a mount and attach it to your X carriage or effector and it makes life much easier. Mostly more for people that move their printers around, normally you shouldn't need to use it often.

A basic volt meter is actually important to have. Doesn't have to be fancy. I'd probably pick one that ran off easily available batteries, there are some that use oddball sizes instead of plain 9V or AA type. Use it for calibrating stepper motor currents or verifying power supply functionality.

Infrared thermometer. You can usually get one that's adequate for $20 or less. While these generally aren't capable of reading a small enough area to give you a good read on your hot end they're generally fine for everything else. Need to know why your prints aren't sticking to your bed? Maybe you're loosing all the heat from the bottom and the top isn't as hot as you think? Also, once you have one you'll probably be finding yourself using it on all sorts of other things.

You could also upgrade your ruler to a nice stainless steel one if the one you're using now is still wood. They're cheap and pretty nice. Maybe add something like a compound square if you want.

I highly recommend Nophead's kit. It's fantastic--the quality of printed parts is excellent, the power supply and j-head are quality, and the Dibond helps to prevent resonances, making the printer quieter than ones built from plywood.

Don't over-tighten the nyloc nuts--it's very easy to do, and doing so will bend the Dibond slightly or crack the plastic parts.

Follow the directions very carefully. Everything is explicit in the instructions, but there aren't as many pictures/videos as I needed, and I ended up doing 2 steps incorrectly as a result. Nophead sells individual replacement parts, though, and everything he sells is quality and has a low profit margin. If you're not in the UK, though, shipping can get expensive.

You're going to need to strip very small gauge wire (for the ribbon cable, among others). Here's a great tip I learned to strip small gauge wire without breaking the wire:
https://www.youtube.com/watch?v=yM4JsxirXPg

As others have stated, get a caliper. Also I find that a dial gauge is useful in leveling the bed--one like this:
http://www.amazon.co.uk/Silverline-196521-Metric-Dial-Indicator/dp/B000LFYK0A/ref=sr_1_1?ie=UTF8&qid=1396249675&sr=8-1&keywords=dial+gauge
Basically, after you get your first print, print up something like this:
http://www.thingiverse.com/thing:92852
Change a setting in your firmware to allow the z-axis to go below '0', and attach the dial holder to your x-carriage instead of your extruder. This will allow you to get a very level bed. Bed leveling is everything--it will help adhesion and make your prints much more reliable.

After you've got your bed level, print replacement parts from ABS for your extruder, especially the gears and extruder block. I had problems at one point with a jammed extruder, and the resulting stresses cracked my extruder block. I had a PLA extruder block replacement, but the way the extruder mounts to the block on the Mendel90 (directly with screws, rather than using an aluminum mounting plate on the j-head's groove mount) meant that it got soft when the extruder got up to temperature, causing the tip of the extruder to wobble and the prints to go badly. Printing an ABS extruder block solved my problems. It's nice to have a backup so you don't need to rely on /r/reprappif or on friends.

You won't go wrong with Nophead's kit. I've seen many kits, and helped many friends build their printers at my local hackspace. Nophead's kit is the nicest I've found. The only other printer in the same league is the Ultimaker, and it's quite a bit more expensive, especially if you want the one with the heated bed.

Once you've got some experience under your belt, order an extra Nema17 motor, and get an extra D-Sub connector and custom PCB from Nophead, and purchase an E3D hot end. Build a 2nd extruder--the Mendel90 allows you to replace the extruder by unscrewing only 2 wingnuts (no tools necessary!). The E3D hot end can print nylon and other hot materials. It's a bit longer than the J-head, so you'll lose some space on the z-axis. But it's a fantastic all-metal hot end.