before investing anything take a look around in the acoustics section and figure out how to treat your room on the budget
before jumping into 5.1 I think it'd be wiser to start simple with a 2.0 setup, most studio monitors are sold invidually so you can just add 3 more to your pair, or buy 5 at once and buy a powered sub.
I'd probably take a look at Yamaha HS50M or HS80M they also make a powered sub that goes along with either.
JBL LSR2325P and LSR2328P are also nice and again JBL makes a powered sub that goes along.
I can safely say that neither of those would be an option you'd regret and once you get them listen to tons of material that you know well and learn the speakers well before you mix anything.
In addition to room treatment make sure you have decent monitor stands, I like using decoupling isolation pads between the monitor and whatever they are resting on. Also buy the best balanced cables you can afford (I'd suggest Mogami gold, if you are able to solder go to redco.com you can buy cable in bulk and all the connectors you need the DIY option is much more affordable than buying premade cables).
So all in all to summarize make sure you budget for the following:
-I suggest the Yamaha and JBL monitors/subs (just pick which works for you best)
-I suggest treating your room (this can be a DIY project and can be inexpensive)
-I suggest investing in the right cables (Mogami Gold Quad/redco.com for DIY)
-I suggest monitor isolation pads
-I suggest good monitor stands
Best of luck to you