Bicycle Mechanics - Rebuild

I recently got tapped by a pickup the other day and bent my back wheel on my trek4100. Here is the proposition
I am changing the wheel set and want to install a cassette replacing the freewheel. I ride strictly on the road for fitness and the mtn gears seam to be slow. What is the recommendation for gearing change out combinations.
New to forum and appreciate help? Thanks

So you are building one or two new wheels? If same wheels, just change the tires to sliks and cassette to Shimano 12-27 for an approximation of the road experience. The long ATB rear derailleur will be a bit sluggish. I would stick with ATB wheels for now. 650 pure road wheels would be overkill on an ATB
bike built out for road work. Steve

First, count the number of teeth on each cog of your current freewheel to establish a baseline. Second, among these cogs, identify the largest one you would ever really need for a serious climb at the end of a long ride. Third, ask yourself whether you really need a higher gear than your smallest cog. Finally, use all of this information to select an appropriate range of cog sizes for your new freehub.

If your largest chainring has 48T, a 13 (my choice) or a 12T cog should be the smallest you need. If it has 44T, consider a 12 or an 11.

Does the 4100 come with an 8-speed stock? If so, you might need more than just a new cassette. New shifters to start....

8-speed road cassettes are hard to find, I hear.

The 4100 came with a 7 speed freewheel. my plan is to buy two new wheels and convert to either 8 or 9 cassette. The crank is a Sunmtour 48-38-28 and the original rear cogs were 13-34.
I am thinking of the Ultrgra 9 with 12-27. new chain and new 9speed shifter brake. Would this be a good set up or is there a better ratio. Love the bike and believe in if it breaks or wears out----upgrade.

I'd hit it.

Lets see, new wheels, 9-speed ultegra cassette, new chain, shift/brake levers.......hmmmm

Have you considered buying a new bike? A good friend of mine picked up a new specialized something or other that was a left over model for $299. If you have a freewheel you bike is old. why put the cash into it. Maybe search around for something used. Just suggesting, not bashin'