I am considering developing a web service based on FMU, which would be server colocated. Can I log in as a regular FM6 client to FMU to work with the database like I normally can with the basic FM6?

Is there a limit to the number of FM cients to FMU?

Also, are there any general rules to determine how powerful a machine (Mac) you need to handle a given amount of traffic?

Yes, if your files are shared. I guess up to 5 clients will work with FMU serving databases.

My last Mac was first iMac, I really do not know.

You still didn't give any indication of amount of traffic.

One of the departments at an old job I worked on used an iMac to serve a web-solution for the State of Nebraska, in additional to one or two people updating the solution locally with FM 4.

I would guess that like anything you need to do some testing and traffic analysis to determine the exact needs.

In most cases low end machines are more than adequate for web serving/Filemaker Unlimited needs, however you may be running a very DB intensive site or have alot of visitors and may need a single more powerful machine or a RAIC.