Hey Michelle,
Congrats on the 2* firstly
if you're buying 2nd hand then absolutely you can ask to take it for a demo for a couple of hours. No seller should have a problem with that. You'll know yourself by looking at it if it's been looked after, if the fittings on the inside look relatively clean and unrusty, if the hull is covered with scratches etc.
For 2nd hand boat I'd expect to pay 1/2 - 2/3 of full price if under a year old and in seriously good condition... less if older or battered. When you get the pictures ask the seller exactly what model it is, then look up prices on shop websites to compare. coast and creek in ballymoney might be your closest.
Another option if you've recently done your 2* might be to ask the instructor or someone in the club / centre where you did your training to come with you and take a look?
Don't know a lot about sea kayaks myself so not much use to you there.
Muireann