Perhaps I'm not the most qualified person to be answering these questions, but I do know a fair amount...
1) Just so that you don't have to start thinking up figures on the spot; develop a sure-fire pricing structure that covers absolutely everything. I doubt that pricing each job individually would work too well, if it didn't follow any structure at all.
2) Please clarify the term "problem customer".
3) Since this is a part-time practice, selling parts, accessories and the like is up to you. But since you mentioned that you don't want to invest TOO much time into this, I wouldn't highly reccomend it. It might not hurt to keep a select few items in stock, if you feel inclined to do so.
4) Promotion - Since this is a part-time thing, you won't have to do much in terms of marketing other than what you mentioned in your thread. Taking out of a small ad, or a spot in the classifieds of your local newspaper would take care of any other voids in your customer base.
A website would probably only be worth your time if;
a) It was listed on your business card & footer of your email messages.
b) You found a super cheap, reliable host for it.
Hope that helps!
- Brandon
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