[Mr V]

I agree with Sand Drag. If you use a lot of pop rivets like many teams once you have a pneumatic riveter you'll wonder how you managed without it.

As far as more expensive tools a lathe and/or a mill would be at the top of my list.

A welder would be at the bottom of my list, there are much better ways for an FRC team to join metal. You need someone with good skills to get good welds and fixing a poor weld that breaks at a competition may not be possible at all venues and even if there is a machine shop with a welder how long will it take. What if you've got a welded frame and one of the members gets bent? Drilling out a dozen or so rivets, putting the new piece(s) riveted back in will be much quicker and something that can be done just about anywhere.