Personally, even if it would be a resource stretch, I would rather my team go with a custom frame. Here's my reasoning.

Engineering experience: For a real world experience, you aren't going to be given a base chassis, drive system. You need to be able to design one. Adapting an old one is good, but it doesn't come up very often.

Plus, designing and building your own fram gives you a very real sense of the design phase. If you mis-measure, you pay the price by having a component not fit. By having heavier consequences for design failures, you learn to take more time on the design.

[/soapbox]

edit: also, as far as strength... custom is far superior. Just by hitting another bot at full speed from 5 deet away, we bent in their straight frame, and sent a gearbox flying. It made us feel very bad, and so we helped them fix it.