Most people do the basics themselves. The Bird is pretty bulletproof so personally I wouldn't worry too much about service history. It's all about condition. At 21k the original plugs won't even have reached the end of their service life. Have a look at the oil, should tell you all you need to know. Don't worry about valve clearances - they might need adjusting at 100k if you are very unlucky. Presumably the dealer will be giving it a full service (if not then this is a bargaining point - I would want plugs, oil, filter, air filter, coolant and just for good measure brake fluid changing - last is pushing it a bit!). That lot will save you around £120 ish in parts and fluids. £3.5k for a fifteen year old bike sounds a bit top end even with the low miles. Depends how much value you place on buying from a dealer. Personally, I don't put much value on it!!