Difficulties:
First of all I wouldn't recommend using a 4-wheel base for use as a robot. There are numerous navigational problems that you encounter when doing that. Tank-type treads, three-wheeled, or walking robots are much better (IMHO).

Secondly, building Ultrasonic Ranging is VERY easy (though it took me a month or two to work it all out) and you can do it accurately and reliably for less than $25 in parts (I'm assuming you have NO parts in your junkbox and buy virtually everything from some obnoxious retailer).

Thirdly power is a major problem. With small robots or robots that don't have as much in the way of electronics, this may not be so bad. I have a 486 PC with a 2-1/2" hard disk and so the power gets sucked down pretty quickly. I'd like to replace the motherboard with one out of a laptop. I think that would relieve the problem quite a bit... anybody have a nice 486 laptop they'd like to donate to the project ??? :-)

Overall I'm very pleased with the development of the robot so far. As I mentioned earlier, the project is still in the works though and I'm having a ball working out all the details. Not to mention all the new friends I've met while perusing the newsgroups and web sites looking for ideas and stuff.