I'm sure some of the more experienced members on here could provide further detailed info on some things but here is my 2 cents.
1. Yes these engines are relatively straight forward to work on from what I have experienced. I'm in the middle of a rebuild myself having very limited (only other car I've worked on was a classic mini) previous experience and things are going ok. This forum and the members on it is an excellent resource to have as a beginner undertaking a rebuild.
2. There isn't any major (expensive) specialised tools that spring to mind, just your usual suspects such as spanners, allan keys, torque wrench, socket set etc etc. What I would say though is that it is worth while investing in a few measuring gauges (dial gauge, digital calipers/micrometer, feeler gauge) as when you go to put everything back together you want to ensure that everything is bang on so that you don't have to end up stripping it all down again.
I have a friend who just recently rebuilt a renault 5 gt turbo and within the first half hour of driving heard a knock coming from the bottom end, turned out he bought standard sized bearings assuming the crank had no previous work on it but it had and as a result had to strip the bottom end replacing not only the bearings but the crank itself.
3/4. Recommended rebuild parts would include, top and bottom gasket set, various seals, water pump, cambelt kit, main bearings, big end bearings, piston rings, clutch, head bolts, spark plugs, leads and then all fluids. This list might be bigger dependant on the condition of some other parts, the only way to tell is to strip it all down and inspect everything. Due to the age of the cars nowadays rebuilt parts are becoming more and more expensive but are still available, just make sure you do your research as some suppliers charge through the roof for some things.
You could easily rebuilt an engine for less than £1000 but it all depends on whether you want to spend the time acquiring parts and doing the actual work, which believe me can take a bit of time , or just get the rebuild out of the way quickly.
5. It really depends on how much extra performance you would be looking for. For instance my engine I'm rebuilding has just been away at a machinists getting the bottom end balanced, a bit of head work, liner seats re-cut and faced slightly to up compression and some other things and all in all came to around £600. This work, although not all essential to up give extra power should see a smoother running engine which is a good place to start when tuning I believe. Then you could talk about adding throttle bodies and aftermarket management etc etc which would add on another wod of cash. As I say it is a matter of what you want out of your engine.
Hope this helps