You need ventilation of some sort. You also need to be aware of anything that could catch fire within five or six feet of the table. Grinding is more likely to cause a fire than welding.
The best surface for the table itself will depend on whether you're going to be pounding on it, or need it to be extremely flat for precise alignment of pieces. Budget is also a factor, of course.
I have four tables I weld on -- which is overkill.
One is small and portable, with a 3/16" top. The only clear picture of it I've got is from when I was building it:
I've since rounded off the corners. It stores away next to my center bench with a cband saw on top of it and a table saw underneath it.
One folds down from the wall and has a 1/8" top. I keep clamps behind it:
One hangs up on the ceiling over my car like the sword of freakin' Damocles:
It's also 1/8" thick, but it's moveable and handy to use for bigger projects, like this fence:
One has a 1" thick top, a 10" vise, and is good for hammering: