Frequency of replacing parts

[uk elephant]

Wheels (bearings are worn out)
If the bearings are worn out, then the hubs should need servicing, not the wheels replacing...
Rear Derailleur
What exactly did they say was wrong with it? And have you actually been having trouble with shifting? The jockey wheels on RDs do need replacing from time to time, but the whole unit rarely fails unless it's been knocked hard/crashed.

My opinion? Get a second opinion, and bear in mind that you could do everything except the hubs by yourself at home with no special tools other than a set of allen keys. And a little patience.

The bike shop back in essex had a look at my wheels a couple of months ago and said the bearings and the hubs were worn and it all needed replacing. They tightned things up to last me for a while, but indicated that next time it would need to be replaced. The shifting isn't good, but I think that's mainly due to the stretched chain.

The guy at the lbs here in Aberystwyth, when he had a look at my bike, jiggled the rear derailleur indicating that it was loose and something wasn't right and that it needed replacing.

Unfortunately, I now live in a part of the world where second opinions would involve a 3 hour drive over snowy mountains. Not really feasible. But I have found someone with links to the local cycle club so maybe someone there has a mechanical inclination and could have a look.

And I will do some on-line research for prices too. I'm quite happy to fiddle around with an allan wrench, but I'm afraid I never have any luck adjusting gears, and I have never changed a chain before.