Investigate what your MLS requires before spending ANY money.
Let the MLS be your guide. You probably have an appropriate computer already . . . . that'll be the least of your expenses. Too often, people think that throwing more money at this business by buying the latest and the fastest will give them an edge when it's still a "people" business. I don't think that will ever be any magic pill.
Once you become familiar with the functionality of your MLS, you'll be much better equipped to devote time (and maybe money) to obtaining whatever will best exploit the tools that your MLS makes available to you.
I'll say it again: Let your MLS be the guide. I'm writing this on a 2002 Desktop still running XP; but whose Operating System I've upgraded several times, added Memory, changed out the Cooling Fans and Power Supplies, changed the CPU Heat Sink, replaced Hard Drives, the Scanner, and the Printers, and Monitors, and the UPS (back-up) a few times. I limp along somehow; but have never felt that I "needed" a new computer to perform my work. I don't think you will either.
_________________________
Dale C. Hittle of GOLDEN RULE PROPERTIES in Glover, Vermont
Where We're Always Striving To Put Together "THE FAIR DEAL"