I love discussing about game design in depth, so no worries about the wall of text haha.
- About the lack of info on the shop, I feel like it's a fine line on encouraging discovery and making the player feel lost. The fact that the enemies use the same parts as the players places pressure on the player knowing what each part does, so they can properly counter/build around it. I played for quite a bit before I got the chance to get the +150 health legs and finally understood why some enemies had so much more health than others. Maybe having an optional way to see detailed stats on the shop (like pressing a key while hovering) could be a way to let the players learn while not overwhelming them? I think the game only really clicked for me when I found out I could see the detailed stats for the equipped limbs.
- On a related note, I only found out about this by reading the tips, since that menu is a bit hidden.
- On the reroll chance, I'm a bit biased because as player, I like to see all the relevant stats before making a decision. Having some kind of approximate indicator of the chance for a bad roll could be a middle ground that doesn't give too much away, but communicates there's a risk and how high it might be.
- About starting the game on the shop, I think that was no problem at all after I learned the mechanics, It was only confusing on my first few runs. Maybe having an introductory battle for the first time players could help making things clearer. If you do plan on expanding the game, you'll also might have to consider that the player is very encouraged to reroll as much as possible on the first shop screen, as there's no investment on the run yet.
- After playing a bit more, I do agree knowing the future opponents wouldn't help the decision proccess that much. Having the player think up builds according to his opponents could be an interesting path to consider though!