Just an FYI, the Spotted Congo puffer and Pustalutus are not going yo be cheap fish to buy... Especially since Spotted Congos like being in groups, so you may find yourself getting 3 or more which adds to the cost... last time i saw spotted congos they were around USD $200 - $250 a piece...
Amazon Puffers (C.Asellus) while not as expensive, also like also being in groups, only catch here is they are known to have overgrown teeth issues which may require manual trimming so be aware of that...
Finally, the MBU and largest of freshwater puffers, they need seriously large tanks to be happy long term, so the fact you put “cheap” infront of MBU is a bit contradicting, since their needs and food bill etc will be far from cheap long term...