long-term goal: qualify once for the WSOP (win obv)
the only qualifier i can play is a turbo-rebuy on stars.
any fundamental do's and don'ts for a tourney-donk?

double rebuy from the very outset. play aggro, try to build a large stack during the first hour, then slow down a bit. don't disregard the add-on.

When i played on stars i used to do well in the 3r to nightly i found re-stealing well late to be my best tool in cashing these
pick your spots wisely and look for situations where you can win with w/o a SD or pick up some added value in the pot with a weak limper who perhaps has a nice 14k stack but uve got the 9k stack and its likely that his smaller pp or SCs etc fold
Playing too tight late will kill your your FE
Consider more than just FE that uve got but what it would do to their stack to lose
to you that implication is sometimes lost on donkeys but once again, player profiling and stealing are imo the crux of consistently cashing trbo sattys

as kaze said add-on esp in the trbos even if ur CL

make sure you are correctly bankrolled!! If you play them correctly you will see some heavy variance

thank you!
is there some kind of rule of thumb regarding risk/reward-ratio when you stop rebuying because you've already invested too much (e.g. stop loss 4 rebuys)?
this probably depends heavy on the payout structure.
same thing for the number of bb you're rebuying. lets say the blinds are 50/100 and single rebuy is 1000. it doesn't seem profitable to rebuy 10bb which basically limits your action to shoving. or is it?!

Always rebuy imediately, always take the add-on unless you have so few chips left that you are waving the white flag. Identify the maniacs, iso them if you can and pick them off. I personally only rebuy 5 times total (including the initial double up). I read that from someone else here and I like it. I find if I've been beaten out of 5 buy in's I'm probably getting a little tilty and this is just not my tourney. I would look at the average stack, to determine if rebuying is prudent. The fewer SAT seats there are the less likely I am to rebuy late, its just gonna be too hard of a mountain to climb and I'd rather just get a fresh start. If there will be a large # of seats available (you can alway project the money pool and estimate the total) then a late rebuy may be worth it even if you will be pushing with it.

When it's late don't get lulled into a false sense of security when you have a fairly big stack. The blinds will get huge as stalling mode kicks in. Some people have no idea how to do this correctly, they actually stall themselves right out of the tournament. Probably why they turn the chat off because it would get pretty vicious. With a lot of seats and a lot of runners that think they are good (whether or not they actually are) push on the bigger stacks late as the bubble nears. They have nothing to gain by calling. Watch out for the little guys and the mega stacks. Former is getting desperate and the latter is obviously willing and able to call anybody. If you are playing for very few seats this is less of an issue. Just play your normal game. I'm not sure which rebuy SAT you are referring too. Personally, I think the best route to get in the WSOP (and the one I persue in my spare time) is to SAT into the $650 qualifier, or collect enough $W or $T from other rebuy sats to buy into it. The step route is just too hard to avoid taking a beat early in one of the rounds to climb all the way up IMO.