I've got a few of the WMS Dotmation machines from the 90's. They can be had for reasonable prices ($500 or less) and also have swappable game kits. They share a lot of design/operation with pins so debugging them isn't completely foreign. The only real downside with them is the display is the large format which makes them pricey to replace if that is ever required. They are actually pretty fun. Clay used to have a section on his site with gameplay video and the like. He did a video episode as well.

Slot machine forum that has a bunch of good info...
http://newlifegames.com/nlg/index.php?PHPSESSID=3b5ea9c4c6d5d504899df5ce6442d32b&action=forum

Best of luck!
Jaz