You don't say how big these fabrications really are. For snagging welds and such in tight areas, I really like a band grinder, here's an electric-powered example:
For taking off small rivets and rusty machine screws, I used to use a Foredom flex shaft tool, that'll get in a lot of tight spaces. You aren't probably going to get one new for what you want to spend, though. A Dremel just isn't the same.
I would assume the low-speed versions of the machines you reference are for running flap wheels and the like, the high speed versions are for grinding points.
Since getting a compressor, I use pneumatic versions of the above for most work, the mini-die grinders that are like a fat pencil are really handy and don't heat up when used continuously. Also extremely easy to get speed control with them. No motor brushes to wear out, either.
Stan