First of all, is always and formost safety !!
Not only are you looking at the boat but the trailer as well, do you have a vehicle that can safely tow this boat? you need to be thinking about everything and family too.
Once you decide you really want to expand your fishing onto bigger waters, make sure the boat is big enough to handle the waters you want to fish, rough waters in a small boat can get you in trouble in a big hurry.
When you think about used boats, your first question is, how do I know I am getting my moneys worth? you really don't know, unless the seller keeps a transaction history, services, repairs, all are part of helping you be a little more comfortable when considering a used boat, things like keeping it in a garage or under a quality cover will indicate how much or how well the seller took care of his or her investment.
What are some things to look for in a used boat? The best advise here will be to consider the size of the water you are going to fish first, DO NOT settle for an under powered vessel, or too small of a vessel, a modified "v" hull may be the boat you need, but how stable will it be on rough waters, think about the worst conditions and look at the boat and see if you think it will handle those conditions on the waters you want to fish, same goes for the trolling motor, is it strong enought to move that vessel out of harms way if need be.
What are some plus items? overall condition would be my first, larger than needed trolling motor, deck space, storage space, it really depends on the application your looking for.
Run away fast? you will know, but things that stick out in my mind are things like a clear title, a pushy seller, wood rot, rusty areas, trashy looking boat in general, no title and "im selling it for a friend" are sure ways to make me turn around and say have a good day.
As far as the test drive is concerned, as long as you have access to a good marine tech that you can have look over the craft and be able to give you an honest opinion, then the test drive will be up to you, me personally, if the seller is willing to haul it to a lake that far away, then take him or her out to lunch for doing so or comp in some way you are comfortable with, but in my opinion, this option is the best way to know for sure this is the boat you will be most satisfied with and have the most peace of mind with.
Always keep in mind that when owning a used boat, even after all is said and done and everything goes flawlessly, it's a mechanical thing, it can and most likly will give you some sort of issue the next time out, it does not mean the seller tried to rip you off in most cases, just look for boats that have been well taken care of and if at all possible have the records to prove it too, always have the option to have a qualified tech look it over and offer a solid opinion, in short give YOURSELF the best peace of mind you can for the investment you wish to make.