While I don't claim Onyx reamps 'rock ass' they have edged nearly anything I've ever heard from Presonus (edged not won by 13 lengths)
In terms of bang for buck you probably can't go wrong with RME gear, top of the stack for entry level converters, pre amps are certainly on par with the pack. Project studio (mine) at the moment is based around MOTU hardware, with majority of basic tracking done via Alesis 24HD. Overdubs, solo's, etc. when only in need of a single track or two generally rely on the MOTU gear. While the actual device rotates, shuffles from one to another I generally have some type of mixer in play . . . not for signal to go through mixer to A/D cards but for all the miscellaneous tasks that a mixer can make easier. And there are times I'll use a fader board control surface for mixing. (not something the onyx can do)
'i' series Onyx will include 'interface', only you can decide whether you need the out of the box (i.e. computer) analog options of a mixer. If you are working with other people flexibility of fold back and monitoring (with a current onyx you might be able to configure both a control room mix and a cue mix via two headphone jacks, I don't remember.) Exploring whether foldback/control room options are sufficient is one of the hardest options to determine via specs alone (checking out SOS, on line, reviews might help). It is easy to spend big$ on an audio card just to find that a headphone distribution and/or mixer are necessary as well. And there are people who are quite happy to not have either.
If you are going to be working with more then one person it is hard not to stress importance, flexibility in cue/monitor options . And no matter how fast the computer or drive it is unlikely that latency, when monitoring through computer will be acceptable. Particularly if you are tracking other peoples vocals.
As you appear to have decided to continue using Reaper there is little purpose in jumping on the band wagon. I don't, never have, particularly like PT . . . I use it only when necessary to interface with client projects. If I had the money $10K+ (and real estate for a separate PT installation) I might well 'do' PTHD, if only because I get so tired of the debate and attitude of PT clients (who quite honestly can't tell whether I'm PT or not if we're doing the distance thing) . . . but my guess is that Reaper is powerful enough, flexible, there's that word again, enough to grow with for a long time.
If you have the money a boutique pre amp, to supplement anything on mixer or converter can be useful. In UK the DAV BG-1, Audient Mico (for two channels and some quirky options) are reasonable bang for buck options. True Systems P-Solo, about $500 for a single channel remains one of my favorite entry level pres (comes with a couple of different variations. Based on cost benefit I am OK with the Black Lion Auteur. While I'm not familiar with all the variations of the ART MPA: gold, ii, etc; series for less money then Mico, DAV, True Systems (while subjectively I don't think they are as good) is, somewhat surprisingly, better then I expected it to be. The one I have, picked up second hand, had some low impact mod's, primarily if I remember correctly replacing the stock tube . . . but that is fairly easy to do.
If you are handy with a soldering iron (or know someone you can blackmail who is) the Seventh Circle Audio (SCA) pre's can be cost effective. They come in the lunch box format and by the time you pay to have someone else assemble them they cost about the same as other lunch box options . . . and typically warranty on the labor is problematic . . . but they are good enough that in terms of acquiring pres that mimic famous (neeve, api) topology with lower cost, they are worth mentioning
No single mic is ever going to be 'right' for every vocal variation, having different mic pres that impart different character to different elements of a recording can be greatly beneficial. Generally speaking I don't have a problem running drum tracks, for example, through Onyx pres. I use them alot when tracking rock band live shows (almost never for traditional chamber music). Occasionally they will be used for pop/rock vocals. But even if you are primarily recording yourself having flexibility with 'colors' of preamps can be useful. If you are doubling an
acoustic guitar part (same guitar, same performer) having different pres provide slightly different character can be as useful as EQ and hard panning in terms of finding a place in the mix. (not instead of sound field location or EQ, merely another tool and with appropriate fold back options the slightly different sound can invoke a different, slightly, performance as well. Enhancing variety of arrangement)
As you have gathered from the posts there are a lot of different ways to configure a project studio. Not ever bit of gear needs to be bought all at once. But if you are planning on working with others, either review monitoring options closely or plan on budgeting for a headphone distribution amp (cheap but sometimes an extra $100 breaks the bank)
good luck
