Considering many of us have 50-100 open bottles on the shelf at any given time, you may have to define "without going overboard ".....
I think I'd shoot to have between 4 and 8 bottles in your stash. 4 bottles of scotch (maybe 2 islays, 2 highlands/speyside), 2 bourbons, a rye and something else like an irish/canadian/american whiskey. Within those, you want a mix of stronger/cask strength and smoother offerings.
If you want nice bottles without going $100+ a bottle, I personally would add:
- Oban 14
- Balvenie doublewood
- Ardbeg or Laphroaig cask strength
- Elijah Craig Barrel Proof
- Old Forster 1920/1910 or Blantons
- Willett 4 year rye
- Tullamore Dew 12 or Red Breast Cask strength / 15 year