You have mentioned quite a few items in your posting that can be very costly. Can you tell us what is your overall budget? The Roland speakers MA-8 that you mentioned are not powerful enough even for your PC monitoring usage. Speakers with larger woofers (preferably 15 inch) give you much fuller sounds to reveal the real power of your keyboards. I have PA speakers with 12 inch woofers that cannot produce the low end punch that I wanted for electronic drum set. Besides using mixer and powered speakers, a cost effective solution is to buy a keyboard amplifier with built in mixer. There are many choices of keyboard amps in the market. Roland KC500 has a mixer for 8 discreet line inputs, its 15" woofer delivers very good low end punch and it can be linked to another KC500 to get you stereo setup in the future, its headphone out is very nice for late night practice of your instrument mix. FullCompass has a demo KC500 that sells for $410.91, a smaller demo KC300 with 12" woofer sells for $314.77, best prices you can find in U.S.
I have been playing synthesizers/keyboards for about 6 years and I gradually built up a system with more quality components that last for a long time - avoid buying toys that will not scale well with your future growth. I started out with a Mackie MS1202 mixer but outgrew it in less than 2 years and had to purchase a Mackie CR1604 VLZ - I still have to use the MS1202 as a sub mixer for convenience. The Mackies are very quiet but they are not very cheap. My Roland KC500 has been working for me for more than 3 years at home and on stage without any problems. Let me know how you think about building your own studio.