1. Yes, a Command Station w/o ROMs in good condition is excellent for sequencing outboard equipment. BUT....
2. Unless the price is right (in my opinion, waaaay under $200, even under $100) then it's not worth it. The PX-7 gets its resale value from the Protean Drums ROM. You should be able to find good condition MP-7's and XL-7's for between $200 and $300. You should not expect to lose much if anything if you sell it off in the future.
3. Given the current prices of the more common ROMS, making sure the encoders/buttons/pads work is more important than what ROM is installed. You can always find a Composer/XL-1/MoPhatt ROM somewhere. A good working set of pad sensors is much rarer.
4. Don't harsh on the purple. Put it in a dimly lit room with all the LED's and screens flashing and you'll find it kind of pretty. Even if you don't, it's not the worst feature of the MP-7. What is the worst? [Hint: how often will you want your song to say: "Oh Yeah! Boooyyyy!"