I'll give my take as I'm in the same boat.. using a $50 HF press and want to get more into traditional folders.
If you have the money to spend on a big mill.. get a bigger mill. If you don't, get the small one and make some knives. If you need a bigger one down the road, and cash available, get a big one then. If you really want to make folders, get to it, don't put it off for 2-3 years while you save up for a big mill. In the end, you'll use the little mill a lot I'm betting.
If I would have waited till I could afford a KMG with 3HP and VFD, I'd still be sitting hear with no experience. Instead, I got a 2x42 and rocked out.
** Keep in mind, spending $1500 on a good used mill will also still require tooling.. which can add up quick.. so figure another $500-$1000 (or more) depending on what you plan on getting into. So now you went from $1500 to 3K real quick. That being said, some of that can be spread out over time.
Just my .02