If the beast is a 1999 model then it's probably not a Boxer engine.. it's more likely to be the transverse, in-line 4-cylinder (aka Twin Spark).
The only real bug-bear of the T/Spark engines is the cam-belt. The cam-belt is challenged by a few factors .. the most critical of which is that the tensioner
doesn't really want to last longer than 60,000kms. It's also possible for the balance belt (2.0ltr only) or any of the tensioner pulleys to pack up... but the resulting mayhem usually wrecks the cam-belt.. (therefore the engine) and so all these issues get lumped in together as "cam-belt failure".
Check that the cam-belt and tensioner and all the pulleys (plus balance belt if you have a 2.0ltr) have all been changed recently. It's not a difficult or specialist job (though you do need cam-locking tools) but as you can imagine, it can be a bit expensive and some people either ignore it, only change the belt itself (leaving all the pulleys and tensioner to do another tour of duty
) or sell up just as the job becomes due.
Ideal world it's all just been done (with proof of all the parts replaced). If not, budget for it.
You should also change the water pump and the variator at the same time... not because they fail but because if they start going wonky then you have to have the cam-belt off again...
The water pump is cheap - £30 here - compared to the labour cost of dismantling the engine again. Change the pump every second cam-belt (120,000 kms) and you won't ever have any bother.
The variator is the same.. they rattle when they wear out.. and it's a cam-belt off job to replace it. If the beast sounds like a diesel when you start it up.. that rattle will be the variator. Again, change the variator every second cam-belt and it'll never give you trouble. If yours really is
dead quiet.. you could leave it till next time (variator is circa £100).
Check when the spark plugs were last changed. The T/Spark uses (as you can guess) 8 plugs (2 per cylinder). They're supposed to last 120,000kms since they're platinum tipped.. but 100k could see them starting to die off. They are easily sourced NGK plugs at the end of the day.. but there's 8 of them and they're platinum tipped.. so £80 for a set here. Your previous owner may have substituted normal plugs.. cheaper but not so great and they'll wear out quicker. Go for the platinum plugs..
The engine potentially uses oil. Up to 1 ltr per 3000km is "normal".. Check the oil level every week .. but if the lad you're looking at has a level filled to "Max" that's a good sign. If it's on the end of the dip-stick, then the owner may have let the oil run too low.. You can imagine how T/Spark will deal with low oil levels... The usual issue with a low oil level is that it knacks the variator apart from anything else.
Having said that, some T/Sparks seem to use no oil at all in which case, great. Just learn your motor's drinking habits and keep the oil at Max. It's a lot less painful in the long run.
So.. it's all to do with engine maintenance. Everything else is pretty much bomb-proof. Wishbones wear, rear calipers stick and you may have the odd minor electrical hassle (though maybe the UK climate contributes to the last two). Everything else is "same as any car".