Fun! It's simple, and the VS-esque structure is well trod ground but executed well. There's a reason this gameloop has proliferated to so many games, it's very solid, intuitive and addictive. The game itself doesn't feel finished (and I mean beyond the missing art), it's lacking in upgrade diversity and enemy diversity. But everything that is here works quite well and as intended. The truck-mech design is a lot of fun and reminds me of the 'Methane-Wanzer' from Front MIssion 3. The game is tuned too easy, at least early on-- I got to ~17 before stopping because there really was no challenge or variety to the gameplay.
Absolutely a solid foundation if you choose to develop further. I'd suggest putting some wacky stuff or tough tradeoffs in the upgrade choices to keep things interesting. Some basic pathfinding to help the enemies get around obstacles would be nice too.
Nice work, solid jam game, great job!