Judging from this and previous threads, what you need is a decent marine catalogue from one of the discount chandlers. Try MMS who do reasonable prices and have a 400+ page catalogue which they will send you. Their site is www.mmsmarine.co.uk. Even if you don't buy from them, their pricelist gives you a benchmark when looking elsewhere.
ps a 5in set is too small - go for a dual voltage (12v/240v) 10" set which will still go in the smallest boat but not give you eyestrain!
We bought a 10" Panasonic with remote control and teletext, 240V and 12V from Dixons for I think about £180 last year. We'd prevously had one of their own makes, Saisho or whatever and after 3 sets breaking down after a couple of days, paid the extra for the Panasonic and it's been fine. Wonder if I can connect a radar scanner to it?