So, what wheelset did the shop keeper recommended? How much ?

Some like the ratcheting sound of the hub, some don't but it is not  in any way better.

As for no.2, check if yours is a seal bearing hub, if it is, no point changing but then again, Shimano have never used seal bearings right from their low end till the XTRs and Dura Aces and they are performing well. Reason is, easier to actually service and maintain more regularly. But on the same note, I think KHS wheelsets uses 32 holes hub (pls check) and if it is, I think it is easier to just change hubs as 32 holes are common and you can buy mountain bike  or road bike hubs so easily here.

PS/ Also KHS uses the standard 100mm front hubs. So much cheaper if just get a used seal bearing hub from BBs and relace the rims.

Faster spinning depends mainly on your legs and also the condition of your hubs.

Rim dishing (centering) is the adjustments of the spokes and not the design of the rims or hubs. It's spinning wild means it is badly trued or else your rims 'bengkok' already!

Etc.etc. too 

importantly is the materials used to construct it.

I think someone on this forum had the Hummer, and the connector post bends as the material used is not as durable.

Generally, i reckon the price difference means:
- better parts , or at least better machined
- non hollow parts (where it is not meant to be hollow)
- lighter bike that you are more likely to use vs a hulking heavyweight

just my two cents worth.
tbh, i was eyeing the hummer too as im just a very casual cyclist (round the condo grounds haha)

Er .. Selling mine if ur interested
http://www.bicyclebuysell.com/item/249928/dahon-speed-p8

got 4 bikes already bro