Hi Chaz,
Welcome to the club!
A used Alembic in the $1,500 price range would do you well indeed. For slaping, I think any Alembic can do it, because the electronics are superior. It is a bonus if you get a custom top wood, or a neck-thru for a used price, but do not discount the set necks. They have a special blooming midrange which can drive the EQ to do special things. To explain: If the guitar produces the tone, the electronics can always enhance or remove or embellish it. I think an Epic, Orion, Excel set-neck would do great for you. Yes, original electronics have vol, pickup pan, Bass and treble cut/boost. BUT, for under $500, Alembic sells the AE-2 filter Q replacement package which replaces the Bass & Treble with a filter sweep knob, and a Q switch in either 2 position or 3 position. This will give you the famous Alembic sound of the higher priced models without the price tag.
Here is a thread that discusses this mod:
http://club.alembic.com/Images/393/31125.html?1159930026 Also:
From my own experience, the epic has a wider string spacing than the standard spacing of the series model, but is a bit tighter than the comfort spacing of my Europa 6 string bass. It is the best of both worlds, fast for fingers, pick, or slapping. The Europa has a bit too much space for my fingers, and I need to work up to speed on it, but on the Epic I need no such adjustment - it just feels right for my fingers right off.
Also, don't discount the Spoiler or Distillate models (Elan & Essence too). As neck-thrus with filter & Q, they are killer basses with gobs of great sounds and can be had used for great prices too. Happy searching. you WILL find your Alembic!
Kris