Welcome to the Landy club (you poor sod!
).
Driving them is very different from driving a car. Expect lots of steering wheel movement for not much actual steering, and spongy brakes as standard. Also beware of your blind spot when changing lane. My Landy is a panel van, which makes this particularly critical, since glancing back over your shoulder only reveals plywood! Likewise, when turning right, you'll lose visibility to the left very quickly, so if you're on your own, you need to be aware of your road angle, or you'll find you can't see traffic coming from the left.
There are lots of guides online on what to look for in a used Landy, but the main things are the condition of the chassis, bulkhead, gearbox and engine. I know you said you've already agreed to buy, but there are some real shockers out there, so do take your time to check it over, and if in doubt, don't be afraid to haggle to cover the cost of repairs, or just walk away if it's not right.
The chassis is basically a ladder frame, with various bits sticking out off it that things like the suspension bolt onto. However, these bits tend to collect crud and go rusty, and a new chassis is an expensive affair. Check also the bit of the chassis at the back (rear crossmember). This collects mud from the back wheels, and can suffer badly with rust, although you can get a weld-on replacement for it fairly cheaply.
As Fleecewife says, the bulkhead is also prone to corrosion, and expensive to fix. Almost all landies leak water in, and this can go down the A-posts (front outside edges of the windscreen), and cause corrosion from the inside out. If this affects the security of the door hinges or where the pedals attach, that's an expensive MOT fail.
It's worth trying the transfer gearbox in all positions (both diff locked and not), and make sure everything works smoothly. Lots of landies never get used in anything other than 'road' mode, so the transfer box can seize up. Mine needed a new linkage fitted which cost me all of £10...... plus two days labour to get in at it (eek!).
Also drive round in circles at full lock and check for knocking in the front 'cv' joints (the ball joints that let you drive and steer at the same time). Also check for scoring / pitting on the balls.
Apart from that, it's just a car. Except it isn't just a car. It's more a tractor you can take to work, and some of your friends will think it's the coolest thing in the world, whilst others will refuse a lift to the pub at lunchtime, even though it's raining
.
Hope that helps!