Welcome to the site I'll try and answer your questions and those that I can't I'm sure someone else will come along and help with:

1. Not very rare to be honest. I don't know how many were made but I'm guessing quite a lot. I remember reading Ford sold something like 11,000 XR3's in one year. Having said that, if you come to some shows you're likely to see a lot more fiesta XR2i's than your escort

2. If you like collecting things! They definitely have a following, just check out how many people belong to the XROC!

3. It's too early to tell with them, look at the original mk3 Escort XR3i's they're the sort of benchmark classic for XR's and you can still find cheap half decent ones of them. Alright mint ones are always going to make money, but trust me they're not easy to sell no matter what people say that don't/haven't ever owned them! Particularly with your Mk5 xr3i, it would have to be pretty exceptional to make more than £1k

4. It's a 1.8 Zetec with slightly lairier cams than the other escorts. It's a good enough engine as long as you look after it, same as any other!

5. That's down to your choice! look at some of the cars here, on ebay or whatever and get an idea of what you like. You sound worried about the value of the car? Don't spend any money on it then as you'll never increase the value by more than what you spend on it

6. As far as you want, exhaust, air filter, chip etc. etc. Twin carbs, throttle bodies etc. It's as much as you want to spend! staying normally aspirated I think 180bhp is realistic, but turbo it and 300bhp+ at the right spec

Post up some pics for us too