Look at the folding bikes sold by bikefriday.com
Since your budget permits, consider Rohloff hub.
If you choose not to go for a belt drive, have a chainglider fitted to the chain.
Almost zero maintenance and bikefriday will build to measure.
If you are not certain about long term use, look at downtube.com
Go for their 8-speed alfine model.
Owner, Yan, is on this forum.