Ryszard,
There are several items in your opening post that have me wondering if you might already have everything you need except for the PC.
All modern laptops, even the cheaper ones, have dual video if you count the laptop screen as one of the screens.
Regarding soundfonts - you can use the freeware SFZ+ from Cakewalk (get yours here:
http://www.store.cakewalk.com/b2cus/ProductInfo.aspx?productid=35-CWSZ1.40-20Eto use your soundfonts - no need to buy anything or be forced to use a certain DAW software.
The 7200 RPM drive is no issue if you are comfortable replacing a typical 5400 rpm drive in a lappy with the 7200 flavors available inexpensively from places like
www.techforless.com and first imaging the old drive to the new one.
I actually haven't found a need for a 7200 rpm drive in my new lappy - yet. Maybe once I get cranking on a big number of tracks, but my typical stuff actually runs happy with 5400.
Can't speak to getting the US-224 to work with Win7. I do now have a Tascam US800 that I have working servicably in Win7. Nice clean pre-amps - much better than my old FireBox. Watch for them to go on sale at Musician's Friend for $99. That price is current about every 6 weeks or so, for a week at a time so it seems.
I also don't understand the 'actual MIDI ports' comment. The US224 has MIDI in/out on the back - What's not proper about those connections?
The US-800 is USB connected, and has DIN style MIDI ports on the back. Doesn't have all the cool mixer sliders and transport controls that the US-224 has, but perhaps you don't use those?
Most laptops in the 400-500$ range have multiple ways of doing dual video. Mine that I just bought has a VGA port as well as an HDMI port on the thing. I use the lappy screen and an external VGA monitor simultaneously. I put all the static stuff (like the US800 control panel) on the lappy screen and all the stuff I interact with on the external monitor screen. However, I might spring for a nice big 32" LCD monitor and use the HDMI. You could actually afford that in your $1000 price limit. Lappy for $400-$500, VIZIO 32" LCD for $350 at Sam's Club (join if you have to)
http://www.samsclub.com/sams/shop/product.jsp?productId=prod3770254&iid=CRSJ684and still have a couple hundred left over for an interface of your choosing.
Boom.