I don't think you'd regret buying one, for the reasons you mentioned at least.
I first got one a few months after they came out in 2010, it broke after 2.5 years, I picked up an N9 which has done me fine up until last week when I purchased a brand new N900 as I need the 'power' it brings.

Some of the ones on eBay do have defects which could explain why they are so cheap, such as the SIM card issue which prevents the phone from recognising it, hence no 2g/3g; or an issue with the USB falling out.

Working, 2nd hand ones typically sell for £120 on eBay or at CeX, but if you can afford an extra couple of quid, go for a brand new one, there aren't that many around any more which is probably the reason for the price hike.

I purchased from this eBay listing last week £170 with P&P: http://www.ebay.co.uk/itm/Nokia-N900...item2c6cbddbfa

Phone is as described: brand new and unboxed, and he has 3 remaining with just over a day left.