Limited budget is fine until its no longer fine. Start saving and shower at the gym.

Concrete will just wick moisture everywhere that it’s attached to. The framing and any vanity that touches the floor will rot out pretty quickly. An epoxy kludge isn’t a substitute for proper construction practices that include proper waterproofing for wet spaces. If you want the look of a concrete shower, then you can go the traditional construction method and use a micro topping over the cement board walls. You would need to pay attention to proper detailing to the max and have a capillary break installed all the way around where the micro met the existing. Not cheap. There is nothing magic that is cheap and that would provide acceptable conditions for a wet use area.

Creative Tile Eastern CT: I can see that a custom shower base and such can be done for less money than a purchased base. For whatever reason, the details of a custom base seem to be problematic for a lot of people. There are multiple posts here of bad shower jobs - where they leak, the water doesn't drain correctly, etc. I know that an experienced person could do this and do it correctly. But I'd rather pay the money for the base and get that part, at least, right. There are other parts of that Tile Redi system that bother me: As a general practice you don't rely on caulk (even 100% silicone) to make something waterproof. The flashing shouldn't be an option where the wallboard meets the base. Likewise I don't like how the "Tile Redi Niche" product is flashed. The flashing around the product routes water to inside the wall instead of to the outside. It depends on caulk in order to keep the water from going inside the wall. The base of the product should be flashed to move water to the outside. The sides should also have flashing on the outside IMO.

There's only one right answer: silicone at the intersection of two planes. And if your tile guy doesn't know this, he's incompetent.