Have to agree with Samt. Everybody has their favourite manufacturer but they do look pricey!
There's loads on ebay at the moment to suit all pockets but the main thing to me when buying is condition & has it been serviced/flushed properly. Age might be a factor reflecting price but if something's been well looked after you'll soon tell if it's been abused.
Sadly I'm too far away for you to have a look but I have a nice shortshaft Mercury 4.5 on ebay at the moment that's mint condition yet was made in 1986 and is really well made starting at £150 !!!
Have a word with Peter at Boats & Outboards on here, he's really helpful & might point you in the right direction.
Generally the more you pay the more modern the engine. Tohatsu seem to be well thought of & personally I like Yamaha engines but can't afford the one I want yet............Always the way
What size dinghy & is it inflatable/air floor etc