Really neat idea! I really liked the art style. It took some time to understand exactly how the upgrading and what-not worked but once I got it it worked well.
The only thing that really stuck out to me is that the 50 HP plus-shaped enemies were a darker shade of blue than the other pluses, which made we a little uncertain of how to handle them at first. I figured it out obviously but it was initially a little confusing since there wasn’t a similar colored symbol in the store.