I'm on my second washer dryer (in 11 years) as I don't have space for a separate dryer either. First one was a £300 Zanussi and it lasted nearly 10 years, started leaking at the cold water intake hose which was repairable but wanted a change anyway. New one has been going fine for over a year http://www.comet.co.uk/shopcomet/product/539236/SAMSUNG-WD8704RJA?cm_sp=FeatureEnd-_-Washer_Dryers-_-position
The only issue I've had with both are after using the dryer function it can leave fluff in the drain holes in the drum, next wash that then tends to get left on the clothes. Doing a quick rinse cycle with an empty tub seems to help and you could always wipe it out if it bothered you that much (new machine does it a lot less than the old one).
I think all washer/dryers will be condensing dryers so no external ventilation required. Watch out for some models that have a drying weight a lot less than the wash weight otherwise I imagine you end up overloading it during the drying cycle which probably doesn't help longevity.
Oh forgot - the other issue I get is washing bath towels, after a 1400rpm spin they tend to get packed down and if you do a dry cycle straight after you get a rock hard lump not a dry towel, but you just need to take it out after the spin cycle and fluff it up a bit and stick it back in before drying.