Virtually all the suggestions are for enlargers that are quite rare in the UK, apart from LPL. The best choice is a Durst M600 series, these were the most common enlargers, an M670 is a good choice, and as others have said you need a colour head as that's a diffuser anyway. Be aware that not all MF enlargers will take 6x7 so if you wish to print that format you need to ensure that you buy a model that does.

Ian