Sounds like you really have your hands full. Just know that before you begin this project it is likely that you will exceed the value of the machine. I'm not discouraging, I'm realistic.
That being said...
I don't know many fluids that stay reliable for a decade. Drain them all and refill.
The overflow tank is not a must have at this time but can be picked up fairly cheap.
My guess on the lights being blown is that someone ran it with either a dead battery or battery disconnected. This will cause it to put out in excess of 20 volts and blow any bulb that is on (highs, lows, indicator bulbs, brake light, 4x4 indicator).
Majority of the "quirky" things you described sound like they are being caused by the carburetor. It's a bit tedious but the carb needs to be thoroughly dissected and cleaned with brake cleaner.
For their age, those plastics seem to be in fairly amazing shape if you ask me. There are a number of products that you can buy at your local auto parts store to improve them further (search "plastics" on your homepage).
Over heating is usually caused by a stopped up radiator. Disconnect it and soak it over night. Then gently spray out whatever is compacted in there. Be careful not to bend the veins!
The other parts can be picked up on eBay as a lot or there are several guys on here that have a pile of parts for these older machines.
I hope I've been of some assistance.
Good luck on your build.
'14 scrambler XP 1000
35.5" BKTs on 18 M26 vibes
Turner Eagle Axles
Triangle Snorkel Kit
QSC Clutch Kit
Oracle blue halos
JL 6.5 marine speakers and Amp in custom glass box
Custom rear rack and Grille
'96 Sportsman 400L
Bored .60 over, ported w/ Wiseco piston
VFORCE3 reed cage w/ carbon fiber reeds
AAEN torquer pipe
EPI Clutch kit
Flywheel lightened 12 ounces
Bored carb., intake and exhaust flange
Marshall Racing chains
Sunstar/ custom sprockets
HPD Oil injection block off
4 alpine 6.5" in fiberglass boxes
2" High Lifter kit
3" extended swing arm
30" silverbacks with black HD3's
2" front spacers 4" rear
11 Ranger XP
8" on 32" silverbacks