A Yammie (50G or 40V) outboard are true workhorses, solid construction, and worth their weight in gold. I'm also a fan of Tohatsu. The M60 model comes up occasionally for sale, so I'll keep a look out.
As with any used outboard, I would want to see receipts and lots of them. Compression test is essential. Start it from cold to see idle turnover, tell-tale water flow, plus check trim and tilt is working. You can service with new plugs, gear oil, impeller and lube nipple points.
Price-wise, expect to pay at least £2k for something decent.
Some 4-strokes here: http://www.seamarknunn.co.uk/download/usedob/used.pdf
Good luck with your search.