It depends on the width of the wheel you buy; a 15x6.5" +40 will have a dramatically different look than a 15x9 +40 wheel. (the 15x9 would will the wheel well up nicely, whereas the 15x6.5 would leave you asking for 3 more inches of lip.)
On S13s, a 15x7 wheel, +10 in the rear is very flush (might be safer going +15 so you don't rub the inner qtr panel) and in front, 15x7 +20 to +25 would be flush. If you can dial in some negative camber, you can go as low as 15x7 +/-0 or even probably -5... with camber, anything is possible.
