I'm finally getting around to building a workbench for my man/gun room(spare bedroom). I plan on bolting on a small vice for gun work eventually and some reloading equip. I'm going to put a shelf or two on the wall above it with a good light underneath it.

So far I've got it planned at 6' long 30" deep and roughly 38" high. The legs are sistered 2x4 and all other supports are 2x4 as well. The shelves are both 3/4 hardwood ply with supports and I think I'm going to use a couple 2x12s and a 2x6 with a sheet of 3/4 hardwood ply glued and screwed(countersunk) for the top.

Well, at 38" tall you are either planning on a station that you are standing at, or a bar stool seat..
It sounds like you figure on using it as a reloading press. If I'm right in this assumption you may want to think about your seating situation and go with something that is closer to a traditional desk height so that you can arrange more comfortable seating. Assuming I am not completely off base, while sitting you may want to pull your chair in. That would require an open area under the desk.

I set the height to be comfortable for standing work and I plan on making an appropriate stool to go along with the bench.

I used oak cabinet bases from Lowes.  This way I have doors and drawers.  I put a 2" thick MDF top on it with thin oak veneer plywood glued to the top with an oak edge.  This looks great inside the house and is plenty stout for the reloaders.  I have the bases around two walls with a 90 degree base with lazy susan style shelves in it in the corner.  The corner unit is great for powder.  I also have over head cabinets on the non-window wall.  If you can find used cabinets, this will be even cheaper.

This is fast to install, and fairly inexpensive.  I did one wall and a year later added the second for more space.

Two layers of 3/4 ply for the top.  Cover those with a layer of 1/4" hardboard that you can replace if it gets gouged or stained.  Add threaded inserts into the ply for mounting vices, reloading equipment, etc.  Your vices, and such should be mounted to their own piece of 3/4 ply with identical holes for the threaded inserts in your bench.  Threaded handles through the pieces base into the threaded insert in the table secure it.  Thus, all of your equipment can be mounted to the same threaded inserts in your bench top.  

examples




last pic shows the two layers of 3/4 with the 1/4 hardboard top

I like the insert idea for mounting a vise and presses. I think the hardwood ply should be fine for the top as the table won't see much abuse in the mancave unlike my garage benches.

What should I use for a sealer? I'm thinking just a general outdoor wood seal to protect against oils, paints, solvents, etc.

Seal either with a bunch of poly or some epoxy bar top clear finish.  Or, if you cover with 1/4 hardboard like I did, your bench will wear its stains and scars proudly until you decide to replace it.  Outdoor finish just adds UV barriers that you won't need inside.  Hardboard is cheap.