I surf rather than use a sit on top kayak but would expect the kit requirements to be roughly similar. Proper kayaking is another matter entirely with specialist stuff like spray decks etc.
I’d go for a wetsuit as a minimum. If you’re intending to keep it up over the winter then go for a 5/3 (5mm body, 3mm arms) with boots and gloves, but you could probably get away with a 3/2 between say May and late October, if necessary layering up with a thermal rash vest on colder days.
I really rate C-Skins stuff – pretty good quality, comfy and not too dear.
http://www.c-skins.com/region
Fit is everything with wetsuits though so make sure you go to a shop where they know what they’re doing and try everything on.
For guidance, water temps are generally at their warmest around now, but as the colder nights draw in they will then get progressively colder until bottoming out in April time.