1. What are weak points of these cars? I know the engine needs to be maintained religiously, as any degradation of oil leads to run bigend bearings etc, would I be ok using a Nulon oil additive as peace of mind, as well as a decent synthetic oil? Are the gearboxes/diffs any good at handling the "thrash" (big revs), or reasonable power output? Clutch is newish.. standard though.
Yeah big ends are a big problem, also the ceramic wheels on the turbos like to let go. Other than that, often quite leaky from the valve cover gasket, and sometimes front & rear main seals. My gearbox (rear output shaft to be precise) let go after a couple of trips to Meremere, with only around 300hp but several hard launches. Other people have had much better luck though, with well over 400hp on stock gearbox with no worries. The only real problem with stockish hp is the syncros, as already mentioned.
2. How to get a tad more power reliably out of the 3S, keeping the standard top-mounted interwarmer, without opening the motor? I'd assume a decent dump-pipe, with a free-flowing system.. and winding 10psi(ish) into it.. what are the limitations boost-wise of the CT26? Is it able to be high-flowed? Or would it be more cost-effective moving up to the "CT20B"? Or would this result in me picking pieces of piston out of the sump?
Yes, good downpipe and exhaust are a must. The factory intake isn't terrible but would benefit from a good panel filter and improved air supply to the airbox. Boost should be safe around 12psi (check your timing first and set it to 8 degrees BTDC), many people have run higher on the stock turbo and ecu etc without problems, but in the name of reliability 12 would be sensible. It can be high flowed but IMHO it's not that big of an upgrade for the $$, CT20b is an equally good option, and if you can pick up a steel one in good condition for a reasonable price (around $700) then go for it, otherwise you can buy brand new from Revhead for about $1500 I think, which is also a good option. The stock intercooler really is terrible for anything other than open road driving. Think about at the least a fan on top (look up SPAL fans), but you would be well advised to either try and source a w/a unit, or go front mount, IMHO. The internals of the motor are actually quite strong, the weak link is the stock headgasket but I've known that to withstand some decent punishment (18psi for 2 years of daily driving!) without blowing.
3. Handling.. I've been told these things tend to understeer like a whale on ice, how to combat this cheaply? King springs, thicker front/rear anti-sway bar?
Whiteline sell a complete kit that's supposed to be a huge improvement. Lower/stiffer springs, harder shocks, thicker rear sway bar, camber bolts for the front.
4.. more a question for those who have worked on 185's.. Cambelt is due to be replaced, do I need to pull the motor/box, or am I able to do it with the motor in? Will replace waterpump at same time..
It's quite easy to do it with the engine in, you have to remove the cambelt end engine mount which is no biggie (support the engine with a jack or jack stand), the hardest part is removing the rearmost bolt on the upper cambelt cover, once that's off it's a piece of piss, and should take around an hour to do (i don't know what all the fuss is about!). Water pump is also a good idea, I changed my cambelt without doing the pump and it started leaking literally a week later, although I'm pretty sure I'm one of the only people to change a water pump on a 3sgte without taking the cambelt off any other pulleys or having to reset the timing

wouldn't hurt to do the thermostat at the same time.
5. Times.. I am aware that these fat bastards run mid 15's/16's out of the box.. I'm not after a 13-sec beast, but a reasonably fast, reliable 14-sec 1/4 would be outstanding! What would it take?
Well my mate Tim (gtstarped) had one with a stock turbo, a good exhaust, RC w/a intercooler, and running about 16psi managed a 13.9. The stock intercooler is the main reason why they do terrible times from factory, also you need to make sure you shift well before redline as power drops off above about 6,500rpm on the stock turbo and revving higher makes your times much worse
6. Aaaaand.. I want to install an atmosphere-venting BOV. Where do I put it?
on the stock IC you need to cut a hole in the curved pipe on the inlet and weld a flange on. Easy as. Recirc isn't a bad idea though