got a 99 non turbo 20v and not got any of them probs and mines done 76,000k now, id get an aa or rac inspection if you still want the car, but if it has the miles you said it should be a pearler!, i have three kids who regulary jump about on rear seats and all of my stiching is fine, as for rust??? i can only think previous owner must have had dogs or something!, as for the door they are big heay doors on the mk4 3dr, check door pins arnt worn my drivers door is level etc but occasionally you have to slam it harder to shut it, the misfire sounds like an electrical problem, maf/temp sensor? or maybe plugs and leads, a aa-rac check should confirm any probs,
oh yeah stereo should be a gamma unit, mine had no cd, but i got a sony six dic changer leads and interface from mcs for ?136 only took 30 mins to fit, there was no plug in the boot for my changer i was told this was on later models that is why i had to run the cables etc under the carpet to the dash.
let us know how you get on, like was mentioned earlier theres plenty more out there, so if its a bad one walk away