I don't understand, how are you going to build any stand if you don't have access to at least a hand saw and a drill gun? Is there anyone that could help you, or at least have some basic tools you could borrow? A decent square comes to mind
Some stores like some big box stores will cut lumber to length for you, but then you're relying on the skills of the kid that's doing the cutting. Sometimes they are very good, and sometimes not so much

Smaller wood shops will often offer "milling services", and a shop like that will cut your wood perfectly, but you'd have to pay for their time and skill.
Center braces are a pain, almost everyone hates having them, they tend to get in the way during maintenance. In the link I provided above, the author is an engineer, and he did all the calculating, and he tells you exactly what length, width, and thickness of wood to use to avoid having a center brace, even for your 125. If you really want to go with a center brace, consider making it removable in case you need to remove your sump in the future.
Last, if you're that unsure about your woodworking skills, maybe consider purchasing a stand? You may even find one used? Other ideas include using cinderblock and 2x4's, but personally I've never trusted that the cinderblocks won't simply topple over