Every forum I read tends to disregard anything below 3 litre, often saying the 320i engine is underpowered?

The problem here is quite possibly that if you look at the 'good/bad' section of the relevant review on this site, you will find that the 4 cyl diesel and petrol models seem to be the ones most afflicted by problems. Whereas 6 cyl petrols, like (but not exclusively) the 3.0 seem to generally be the most reliable.

A Honda or Toyota would be a cheaper, boring, bet that shouldnt let you down.

Obviously this will be down to personal opinion, but, in mine, a BMW 318 or 320 would be no less 'boring' than a Honda or Toyota. In fact, i'd say in all honesty that i'd consider a MK7 (2003-2009) Honda Accord a much more interesting (not to mention much more reliable) choice than the BMW (especially the 2.4 but even the 2.0). I'd also be looking at the Mazda 6, which is a good looking, reliable car which drives well.

Having run a 320d touring through to 7 years and 80k, I can tell you I had precisely zero issues with the chain or with the engine. Or, to be honest, any other non wear and tear item. Absolutely reliable and was fun to drive. No DPF issues either, despite mostly short runs. That's not to say there are not plenty that have issues, but i didn't (and I maintained it well).

That said, owning one really is death by a thousand cuts. The car 'told' me to go back to the dealers with unerring regularity for this or that check or part change. Brake pads and discs all round every other year (not cheap). Run flat tyres seem to be made of cheese and needed replacement well under 20k, and constantly picked up nails. I bought at least one new tyre every six months for the length of owning the car. Maybe just unlucky. And they are staggered front and back, with wider rears; a full set from a premium brand will set you back at least £700 (not from the dealer!)

The clutch (and DMF, as you might as well do both as the labour cost is massive) was quoted at a whopping £2k from a dealer, which was the point at which I decided to get something else.

I would say all in, it cost me a good average £2k+ per year just keeping it on the road, which is not chicken feed! Great car to drive and economical for what it is, but owning one is not cheap.

My oldest son found the same running a well mainatained 318i. £1k new pads and disks, then new ignition coils etc..

You need loadsofmoney to run an elderly BMW as a daily runner and using someone else to maintain it. Garages see £££s in their eyes when you roll up.

The 4 cylinder engines are not more prone to chain issues. They use the same chain design on both N series engines.

It's just that for every 6 cylinder they sold, they sold about 10 4 cylinders so the issues appear far more common on them.