What about an E-maxx? No conversion needed, and could be a fun project. Tons of upgrades and parts support. Chris did a really nice build thread on his "beater" project. He had quite a bit of money into it, but some of his cheap mods were pretty good. Keep in mind, you could run a buggy on 4s, and have very little driveline stress. Buggies are super tough, and their parts (such as tires and bodies) are cheaper then truggies or MT's. You got along with the Slash you had, and the Rustler, and the rc18. A buggy could probably do most of the same stuff.
Don't let the price shy you away from what you want. Do it right the first time, and if you have to save, go for it... I haven't driven an RC in a month, which is a really long time for me. Hoping the end result will be worth it though.