My Plan Is Simple, Take An Exi

My plan is simple, take an existing robotic platform i.e. Roborover, or omnibot and turn it into a fully functioning robot. I'm looking to add full functioning arms in place of current arms, reutilize the drive train, and retrofit the head with cameras & distance sensors. My children and myself think this would be a fun robot project. After we complete the build our plans get a bit murky. They want to go full ai, I'm a bit more grounded and just want to see how many features I can cram into the platform. Our limitations are in budget & time. The older of the two that would assist in the project has a lot on his plate already with sports, school, & scouts. The younger one has cheer & school. So we would get two good evenings a week, if we were lucky. The budget, well family of 5 single salary. I was figuring on picking up a used robot on eBay in need of fixing, and going for your developer kit. This should keep costs down to the 200-250 range. I hope this gives you a picture of what my idea is.