i'd say go with the mc21 if your average on the spanner cos it'll give you less trouble in the long run when it comes to seeking advice and setting etc. plus you can sell the mc28 bits as a complete engine and i guess someone will be keen to purchase it from you. sell the mc18 aswell and you should have enough cash to buy plenty of working mc21 parts or someone my be willing to give you a swap for the parts needed. if the engine in the mc21 isn't good enough for your needs then maybe the 300 kit will be a better option at a later date when you have the cash.

sadly it's only an opinion...the choice is yours.