To be honest, I'd suggest only going with a professional service like Aweber or GetResponse.
I think both of them have an import feature (but with a daily limit)

If you build your list and relationship with them the right way, by the time you hit the 10k limit,
the money you'll make should far outweigh any additional fees.

I tried free services and "cheap" services when I first started. Big mistake.

I don't know what you're planning, but given your initial statement i.e. import ... no limit ... unlimited
subscribers... broadcast etc, if you're looking to import a list of 100,000 people or so that you paid
for somewhere else, and then blast out to them, you may find that you're wasting your time (and money).

Richard