How much money you need to spend is hugely variational. I really can't even estimate it. Let us know what your goals are for this car.
A 'good' hp level is dependant on how fast you want to go, and the weight of the car plays a factor here too. If you're going to leave it stripped, you might be satisfied with 130 or so hp, or you could be like Bart and want 200+ hp in the lightest car you can get. I'm happy right now at ~170 hp with a full interior and sound system. It's good for high 14s on the quarter, or should be anyway.
There are really quite a few options in the way of engine swaps, and as long as you're not in Cali, most of them are easily smoggable. Downest is putting together a swap article, it's still in prototype stage but it's a sticky in the modification forum.
Maximum hp is relative. The most I would want in a streetable car is probably 250. I drove Jonny Black's car about 2 weeks ago, he's in the 250-260 range, and I really wouldn't want any more than that at all. It's very fast for road use. For a drag-only car you could see 400 or 500, but that's hardly cheap. Reliability becomes an issue at those levels too.
I think that any of the brand-name off the shelf catback exhausts sound great. These have prices that are occasionally scoffed at however. I paid $480 for my DC Sports. Thermal R&D and GReddy make exhausts in this range too. A custom exhaust from a local shop can still sound real good if you use a few resonators, and it'll be a bunch cheaper.
I've never heard anyone say the throttle is too sensitive on our cars. You can adjust the throttle cable, but I don't know if that would fix it for you. the dx engine you have needs to go though, so don't worry about it too much.
Honda didn't offer power steering on the CRX on this continent, and I think almost all of us prefer it that way. It is true that you can retrofit it from an 88-91 Civic, but you honestly don't need it. Adding power steering fluid won't help, because there's nowhere to put it. :lol: