i've laid slate floors in two of our houses, the current house it is all the way through downstairs. i find it relatively easy to lay provided you take your time.
as blazin-saddles suggests, definitely grade your tiles beforehand. makes a huge difference when laying, and i also do a dry run to further help me get tiles of a similar thickness. it also helps to get tiles that have been planed on one side (so the thickness is more uniform) rather than riven on both. i've laid both, the latter is harder.
to seal we use hg impregnator, i think its medium priced. always found it good.
to cut i do not use a tile cutter, i prefer a large angle grinder with a decent and new stone cutting blade. maybe a pro tile cutter would be ok, but as per blazin-saddles the cheapos from b&q are not that good.
if you are laying tiles around door frames, its easier to rake a slice out of the bottom of the frame and slide the tile under than faff about trying to cut the frame outline into the tile.
slate itself varies hugely, its worth finding a proper stone supplier and having a look. the quality also varies between suppliers, for instance i looked at brazilian at two places a few miles apart and one was vastly superior to the other. i know some great ones in the west country if thats any use. for the ones you've been offered i would try and work out the m2 cost, thats the only way i can see that you may or may not be getting a deal.
for what its worth, i paid £21m2 for 50m2 of 900 x 1200 and 900 x 600. the slate is, i think, brazilian but its teh same stuff the national trust 'allegedly' use in cornish properties because it is identical to local slate - ironic really that its £20m2 from the other side of the world yet if i'd bought it from a quarry an hour away it would have been £200m2 at least.
one thing all those naysayers who don't like getting their little toes cold seem not to mention is that slate has an incredible wow factor when selling the house, in all the properties i've dealt with its been a real deal clincher.