The worst jobs to do are:
1 gearbox , check for fluid leaking under the gearbox if poss get it on a ramp while you test drive it.
2 rear trailing arms , get behind it and make sure the rear wheels are not tilting inwards at the top as it can be costly.
3 nocking at the rear

could be the bushes where the rear subframe mounts which are not available without the subframe (£500).
4 clutch/dmf (dual mass flywheel) this again can be costly if your paying someone to do it i reckon your talking £200 to do the work and about £300 in parts (double that if you go to fiat).
5 turbo , mine is just approaching 100 thousand miles and is ok but it is letting some oil past it (£450?)
6 Knocking at the front, not so bad once you get over the shock of the roll bar £110 (if required) as most of the time it will be the droplinks which are about £30 and wishbones which can be had for about £30 each side.
These are prob the most common that pop up here but remember this is a small community and people only seek out these forums when something goes wrong so dont let my list scare you off

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I think the Multipla is a crackin car and couldnt think about replacing it with anything else so the smiles per miles are worth it.