I would drop the first three from the list. The stock TB is just fine, and you probably wouldn't need a larger one until after intake and head porting at the very least. There is a better cam available for almost the same amount as the VX3+cam gear, which is the Phase 1 from Unitek.

You didn't even list sway bars, which is the first upgrade I would do to any RWD Volvo. They're the best bang for the buck as far as suspension components go. If your car needs a new exhaust, then the next thing I would suggest would be the 2.5" system from IPD or the 3" from MVP. If the exhaust is good, put money towards other things first. If the engine checks out (good compression, etc.), then a manual boost controller and boost gauge would be a good investment to raise the boost a little.

Your '90 turbo should be the smaller mitsu unit, so you probably don't need a higher stall speed torque converter as much as you would if the turbo produced more lag. So basically, if money is an issue, spend it elsewhere. A better cam, a little more boost, and some chassis bracing. With a stiffer chassis, the stock size tires will begin to limit performance. Consider some wider performance oriented tires if you feel that you need more grip. The Kuhmo 712 is an inexpensive performance tire that a lot of people like. 205/55/15 on 15x6 wheels, or if you have the 16" Hydra wheels, you can probably fit as much as a 225 width tire (or 215s for sure).

Lowering springs will help the handling some, and help the looks some, but they aren't for everyone, and they're kind of expensive. If you don't mind a stiffer ride and you want better handling, then go for it. You'll need to replace the shocks/struts too if you don't already have some that will work on a lowered car (Bilstein, Koni, Boge Turbo Gas, etc). Prices for springs/shocks installed would probably range from $400 to $1000+ depending on which shocks you get and whether or not you pay someone to install them. Consider replacing the upper strut mounts at the same time, as they are likely worn if original.

I know that the spoiler is appealing (I want one too, for my 240), but it's not going to help the performance at all, so I'd have a hard time spending much to add a spoiler. I've done some less expensive exterior mods like changing the grille, blacking out the trim (or removing trim that's unnecessary), painting the taillights (red turn signals), etc.

"My dad says it's not good to install new internals on an older engine,
is this true?"

I don't know. I do know that many, many people install new performance camshafts on turbo and non-turbo engines that often have 100-300 thousand miles already, and I've rarely (if ever) heard of any problems being caused by the mix of old an new parts. When a new cam goes in, the valves should be adjusted. If they weren't, than that could potentially cause a problem, but otherwise I think it's OK.

Good luck.

PS: you might find some useful info and source of parts on my links page...

Volvo 240 Links