I agree with getting your first set of monitors over a new mic.
To me the main HR "hub" is your recorder and playback.
its the canvas of a painter imo.
you mentioned $300 MAX. Thats really tough.
personally I think 6" is the best medium, 5" you lose some bass, 8" you lose some upper mids...theoretically.
if your adding a sub, which most do these days...then its another topic all together and a 5" may possibly be better than a 6" as it can move faster due to its smaller size reproducing the mids better. smaller faster high freqs', larger cone from center to edge for bass,long freq's.
I don't see any 8" for
$300 max....so thats a mute point.
but I assume you don't have the sub. So I'd consider the Samson Resolv as you can get into the 6" range for $300 with the 100watts.
The rest are all 5", less power and loss of the bass, theoretically.
but those Samson Ribbons do look sweet.......
The Room
Just as important as the monitors is the room setup. 3ft triangle / Nearfield setup helps reduce this problem, wall treatments can greatly help too.
Check the 4th picture out at Ethan's-Real Traps.
http://www.realtraps.com/art_room-setup.htm
One thing that is enlightening, on this setup subject/science, and very subjective purchase of speakers, is the physical placement/setup of nearfields and early reflections seems to be agreed upon. (as in Ethans link) Other subjects, it seems, will be debated for eternity.. tweeters on outside or inside or speakers sideways or upward...
This Rubicon manual shows the EE triangle too, equilateral/equidistant, but in much less detail than Ethan's pic. But the idea is the same.
http://www.samsontech.com/products/relatedDocs/Rubicon_OM_v5_1.pdf
Shopping...craigslist.com may save you some $$$ and be local. I've been doing this recently and its pretty nice. there's no Ebay charges and hassle, and craigslist is Free. the deals don't last long usually.
Cash is King. No credit as its mostly private party's and no returns is the downside of course.
good luck, hope the links help!