Hoping to get around $750. Dahon Speed TR modified with shimano 105, etc
The conversion process was in this post below...

Dahon Speed TR drop bar mod +

Changed stock seat to Brooks B17 leather seat.
Changed stock front dynamo headlight to german engineered Supernova E3 Pro Light.
Added superflash rear led light
Upgraded chain ring guard (metal from plastic original)
From flat bar to drop bar modification: Shimano 105 shifter, Shimano 105 derailleur, Shimano 105 chain, and to make sure braking doesn't suffer, problem solver brake agent (x2) to increase leverage.
Upgraded to Salmon brake pads... it stops!

Has 30 speed gearing system (up from 24 speed) means you will most likely find gear where you are happy with.

These are latest picture taken.