If you're referring to Flexirent "The best way to pay for technology" it's only a rental in the sense that you don't own it, as someone who works for them I can tell you it's an absolute scam Our biggest customers are 18 year olds with no clue, and single mums with 10 kids who want to buy a 60 inch plasma to watch daytime crap on.
On topic though, have you looked at some cctv type solutions? Don't alot of those do multiple channle and HDD recording now? might be an idea to see if you could pick up a second hand/cheap system....