What percent of spanish do your does have in them? If they are 50/50, I would go with a heavily boned boer buck, But if you are wanting to keep more of the hardy in them and beable to range them more and not so much creep feeding then you may be better off to go with spanish.
I guess the other option would be kiko, not sure if you are wanting to add a 3rd breed into the mix.
Feel free to PM or ask on here about how to go about buying a boer buck, what to look for. There are a lot for sale right now, you may be surprised at what you can find for a good price, even with papers and ennoblements in the pedigree. That would help increase your sales, if you have extra doelings that you don't have room to keep they may sell better with papers and you will beable to ask a little more for them.