Valve stem seals is a 40 minute job - I generally do this on any CVH which is eating oil, it's not expensive. Just use it as an excuse to get a better price. Camshaft too - you know that something better will be going in eventually so use a rattly head to your advantage. TBH I'd be more worried about rust and the quality of the interior, also things like uneven tyre wear, CV joints, gearbox selection etc.