The cash shop is not a flaw at all. You can get all of those in-games legitimately. Even if you wanted to drop $100 on a super founder pack, you should never buy frames or guns on the cash shop. They are overpriced compared to how relatively easy they are to get.
Except Vauban I guess, because his parts only come from alerts. If you really want to be Vauban you could buy him, but you should be able to get his parts from alerts in a month or two. I got all of mine in around two weeks, and that includes missing a helmet spawn. The three alerts you'd have to do take maybe a half hour tops, and they post alerts to twitter
(and there are programs that beep and show stuff in the windows system tray.)
What is worth buying in the cash shop is extra warframe/weapon slots. This doesn't effect you in-game, but it lets you have more than 2 frames and 4 weapons (or whatever the default is) to pick from out of game. They are pretty cheap, and it's a convenience feature. No gameplay impact.
It's also worth buying orokin reactors and orokin catalysts. They come randomly through alerts and the devs force one to spawn after every livestream, but you more or less need them. They are ~$1.5 each without any discounts, something like 25c with the starter packs or 75c if you get a -50% plat cost login reward. You only need one for the frame you choose to play and one for each weapon (totaling 4, or about $6). Obviously if you want to seriously use more weapons you need more, but the game has to make money somehow. So you either wait on the alerts or livestreams, or you buy those.
It could be worth buying a pet if you are new and don't want to farm the materials, they come double potatod (orokin in the pet, orokin in the pet's weapon), but it really doesn't matter. You will be swimming in credits in no time and the material cost for sentinels (pets) is not that high. I'd recommend against it.
On the orokin catalyst/reactor front, update 9 is going to redo alert rng so you should be able to obtain them in-game a bit easier without the cash shop.
The levels are all randomly generated each time you play. The objective isn't random (although sometimes it changes mid-mission, this is based on chance). The tilesets and room count for each tileset is fairly limited right now.
It's pretty easy to aim in the game, although I rebound duck to space and haven't found a super preferable jump key. I use mwheeldown to jump right now, but you can't do wall runs with mwheeldown jump so I hit a key on my keyboard for wall runs.
There is an exp item in the cash shop, but I don't know anyone who has used it. You can level things to max in 4 hours or so once you are established. This is important because you can reset a weapon or warframe to make one of the card slots have reduced cost for certain card types. It's pretty much a pve farming game, so exp flies at you while you are doing other things.