Fuji:
Good luck if you decide to go for it, and your reasons for wanting to do it could be regarded by some as being on track for making you a better instructor than someone who is 'only' doing it as a step up to an airline career. If money is an issue, then consider that it will probably take you several YEARS as a part time FI to earn back what the ratings have cost you in the first place.
To answer some of your questions:
1) That would depend on your colleagues and the CFI. Most part timers get the trial lessons to start with and then gradually build up their own client base. If you were to be allocated some existing students against the wishes of the full timers then that could be a problem.
2) Most schools are unwilling/unable to employ enough full timers to cover for their weekend demands, so part timers play a big part in providing for the weekend peaks. Some schools will pay a lower hourly rate to part timers but charge the customers the same.
3) You don't need an IR initially/at all, and you can decide to get the night and IMC restrictions removed at a later date. More cost, however, and you may find that the full timers have the night and IMC instructing sewn up so you will struggle to get into it. As for a multi-engine rating, it's not a lot of use as you will need to gain quite a few hours on it and then do a multi engined instructor course (more money!) before you can earn from it. Again the full timers might have that sewn up as well.
4) See my preamble.
5) You would have to convert to UK ratings on your return. More cost and time.
Good luck with it. I have done part time instructing for several years now and really enjoy it. The pleasure I get from watching the penny drop is reward for all those days looking out of the window waiting for the cloud to lift or for the phone to ring!
