There is a guy that bought a G704 & got rid of his IH clone mill. SO there are the few that said they have to big a mill.
Your right it is better to have a mill that is outfitted with the needed tooling, compaired to having a bigger one without tooling. But in your case I feel the bigger mill will be worth the expience. The conversion cost, or the tooling cost will not be much different between the two mills. It will mainly be the diff in the cost of the mills.
IMHO just looking at the G704's specs I believe I would go with the smaller mill. The smaller BF mill will be compairable to the G704 Grizzly mill & if you have watched Hoss's build thread of his 704 you will see the machine is quit capable. I dont know if its possible but get a R8 collet spindle if you can. This will help the ridgidity & help make getting tooling cheaper, and easier to find good used tooling.
A bigger mill is always nice, but the budget does have to be considered. After all the price of tooling & software will always cost way more than the mill itself. If it can be afforded then the bigger mill is just about always the better bet. BUt if the budget is tight then getting a machine up & producing parts will just about always pay for itself if one does any work for outside money. If the opportunities show themself, then chances are that getting a second machine becomes very possible because of the money generated from the first.
Ofcourse if only work is done for ones own pleasure & needs then this want happen. Also you have to consider if there is room for more machines, plus will you be able to afford upgrading if you do end up needing a bigger/stronger mill later. If you have to buy once & be stuck with your choice then think about your purchase very much. It may take longer to outfit a bigger mill, but if the finds will be there then it sould be considered.
Another thing that will happen is if its the first machine just the learning curve along will end up costing more because of broken tooling & mistakes/crashes. Start out with cheaper endmills & after getting more experiance move up to better tooling. The better tooling will always pay for itself in performance & longer life, which is also easier on the machine. But breaking many endmills in the beginning is something to expect.
Jess
GOD Bless, and prayers for all.