If they are going to be the first 3 level in the final game game, they are difficult. As mentioned Magistross, when you start purchasing it is easier, but the problem is that you get all improvements very quickly, and the game it is not the same. But I like the on map shop idea. Another option would be to unlock only some upgrades on the shop when you complete a level, or simply using a traditional lineal method like in Xenon 2 (level1, shop, level2, shop...) without a map.
Maybe you can make the game easier, using the analog sticks and the mouse position (Tyrian), and lowering penalization when you collision with the scene walls.