Re buying a mk 3, 200 is a fair price if you're in the land of the fee, I mean free, for a decent not abused rifle. no1 mk3's have no inherent fragilities, and a couple of things show up with age. Make sure there are no cracks on the forend going forward from the big screw at the front of the trigger guard and that this screw clamps firmly on the wood, ie that its not firmly tight and the wood still moves a little for and aft. This is a recipe for splitting the for end dramatically from recoil, one shot will do it.
Otherwise; its always good to know that the headspace is OK and what size is on your rifle, ie .065 is average, and I mean know what it is, not take someones word for it. A nogo gauge usually runs $25 or less on ebay. If you're buying you might be able to borrow one or watch it done by the seller or friends.
Otherwise, old enfields were all used with corrosive ammo, and not all were looked after, the bore should have rifling and not be dull pitted rusty etc. Dust on the other hand often hides a perfectly good barrel, so a push through with a patch can be rewarding. Otherwise, I always check a 303 bullet or dummy round point first into the muzzle to check for wear. If its worn out ie the bullet is not engaged in the rifling, the bullet will go right in up to the brass of the cartridge. If its in great near new shape it'll be about 1/4 - 5/16ths from the muzzle to the brass. Anything in between I take to be a measure of half life etc, its just me, but this works for me, others might have other opinions, ie wear where the bullet starts in the barrel, but its not too easy to check that in a casual buy.
Otherwise, check gunbroker.com for ideas on prices and conditions, a really useful way to get an idea if you've got a bargain.
btw, indian made enfields sell for less money, for no real reason, and there are a lot of sporters and jungle carbines converted from no1mk3's, some good, some really not so good, they have a value but their collectible value is mostly gone.

You could always post pics and a description here before buying, all the best with the buying.,