Congratulations on getting over the first hurdle (space)

. I guess you had to do some sweet talking

.
What sort of ideas are running through your head plbab?
Dimensions? Type of layout ... scenic, switching, end to end, out and back????
The dimensions and type will determine the type of benchwork. Others will be more than willing to advise you on that.
Personally, for my HO 18" deep L shaped layout I used 3/8" plywood (which I feel is overkill).
I can understand your enthusiasm to get stuck into construction but would advise you to have a good long think and do a bit of planning first. It is remarkably easy to "dive in" only to find that that this, that and the next thing could have been thought out first. That's a sure way to disillusionment my friend. If you can get your hands on one or two books ... eg, "N Scale Primer" published by Kalmbach Books, you will not regret the investment.
Good luck and happy building, keep us posted.
Errol