Agree with BTM.......
I would never attempt to do the above, your always going to have problems of one sort or another and it'll end up costing alot more than you think as well !
I know from personal experience, been there done that
And yes, you end up with a bastardisation nobody wants, if you ever want to get rid of it, which believe me you will in the end...
I'd prefer buying something for 40k already complete,