+1 for freemium approach to make money.
I also suggest something with a residual income model, if you are going to need to live off this!
http://en.wikipedia.org/wiki/FreemiumAn example of where I think the freemium model works very well is Lastpass.
Lastpass example:
a good solid service for free.
premium costs you $12 per year (so residual income model)
premium gets you access to exta features and a support ticketing system and an sla of some sort
Consider paypal recurring micropayments as a way of collecting your money, that way you are not having to deal with credit card details.
People tend to keep valid card details on their paypal accounts

A web service to have a central store of config would be nice but would not come without costs of its own of course, maybe include this in premium?
Also consider only enabling premium functions if the central web service has a valid paypal payment in the last couple of months. (let the user know whats happening of course) This does require occasional dial home of course... which some will not like.
I really like MremoteNG but bugs (read crashes) stop me from using it for now sadly.
Its human nature to walk away from free products that don't work, but if you have paid you will report bugs and bug reports is what you need to fix them.
I hope it all works out for you.
Long live MremoteNG
Jeff
p.s. Don't feel bad about asking for money for your work.