Dont let your heart blur your vision into this car there are plenty out there and they aint all fetching silly money.
i only knew of a sportline after i bought mine, i bought the first one i saw ?2.5k on a H plate, caused a few problems and has cost a further ?1k to fix wouldn't want another version now as the plastic seats turned out to be leather and the buyer didnt know it was a gti.
few things to check though.
check that fuel pipe stick yer hand under the rear wheel arch and feel for the pipe at the top where it goes through and out the body infact take the fuel cap out and have a look inside, any rust hole either walk away or knock shed loads off.
operate the roof at least a few times costly if it goes down, if its weak at moving open the boot and make sure that the handle is set to the closed position on the electric roof mechanism, if its still jerky and very slow then its on its way out, check the MFA for all the read outs if a readout is not displayed the MFA is not damaged the sender is damaged or been removed, knock more off.
take it for a test drive and give it some stick from 1st to 2nd if you hear a grinding noise, the gearbox needs reconditioning, it wont get any worse but its a bargaining point.
get in the back off the car with the roof up and check around the rear window frame, lift the rubber and check for rust if there is it needs fixing otherwise in time your window will fall out.
outside looking at the roof where it meets the body the edge, roll it up a little it will move up a little enabling you to look underneath, look for rust all around here, its the point where the roof is fixed to the body if its damaged it is difficult to put right.
check the horn and the indicators, the washer wiper can stick as the contacts are crap on them.
