HP becomes important when dealing with the pump and blower. The HP# itself really doesnt mean much (well it does but bear with me). If you are using an industry standard 3cp pump or equivalent a couple of HP is being used to turn the pump at speed. Each blower is going to use a different hp@?rpm to achieve the desired lift@cfm. You can find the lift/cfm performance vs. hp graphs at the blower manufacturer's website. In this case the unit you are considering is using a Roots 33 blower. Info on the blowers can be found here. http://www.dresser.com/internet/businessunits/roots/docs/products/S-SRBS%20rev.0607.pdf. 

Unfortunately there is more math involved than I care to get into @ 4 in the morn. Things to consider are the rated HP vs the RPM the engine will be running at. You dont want to run the engine at max rpm so the actual HP is lower than stated at the rpm you will run it at. Just for example  say that the engine is rated for max output of 50 hp at 3000rpm but you will be running the engine at 2000RPM at which point it only develops 30HP. Now You have the actual HP.

This number is important because its the number the draw of you other components (pump/ blower) will be alloted. For instance now that we have the 30HP number if we were to add a pump that used 15HP and a blower that used 20HP (made up numbers) for a total of 35HP our 30HP motor wouldn't be up to the task.

Then you have to account for the fact that the HP numbers you find for running pumps and blowers are based on the instant HP/torque of electric motors. The HP required to run them on a gas v-belt setup is actually more. Then you have to consider the rated output vs. what you will actually running.

These would be things to know if you were designing your own. Since you are buying a system rest assured the engines on the units you are looking at are more than capable of running their attatched blowers.

The much more important question is the performance of each units blower. Unless you are pressure washing any of the pumps will be ok. The HP increase you see on each unit is because of the increased HP required to turn the blower. I cant see what blower the 18HP is using but the 13HP uses a #33 and the 15 HP is using a 36. Although the 36 produces more CFM it is at the expense of lift (12" vs. 14" at reasonable operating speed. While on the other hand  the #33 provides 14" of lift it really lacks cfm. Honestly I dont think you would see $3000 worth of difference between the two. If it were my money I'd go with the 13HP.

Having said that I wouldn't buy any of them. Given that the biggest complaint among customers is dry time the last thing I would want is a weak blower. I'd find a good used TM. Carpet cleaners with nice units go out of business all the time. Look on ebay. On craigslist right now I know of 4 much better TMs in in your area in the same price range (shopping myself) assuming you're not financing.

Bottom line is you want the most CFM @ 14+" you can afford. Anything less than 200CFM @ 14" you might as well save your money and work with a portable till funding is available for a proper unit. Not to say you couldn't do the same with the units you are looking at............but ....

sorry for rambling