You can get started by repurposing a computer you have and just adding cards. As long the motherboard has PCI-E slots and at least 2 GB ram it will work. Add risers and maybe a bigger PSU and you are golden. You can also find many used Dell/HP towers for <$100 on Craigslist or at Goodwill. Here is a good thread to read for other ideas.
https://bitcointalk.org/index.php?topic=1955358.0Yeah, I think that is going the right direction. When we started we first just started adding cards to a desktop, the main thing was to get up an running. Personally, we started buying cheap cards off craigslist and facebook market place. There is always someone that needs money. We ended up with 100.00 970s, you just have to haggle a little bit. Even at today's prices its only about 3 month ROI. You will never see that with new cards.
Start looking locally for used gear CHEAP, then gradually start adding. I'd bet for 400.00 you could have something going.