From my own perspective, for their intended purpose and for the purpose of the OP here in the 'newbie' forum, both the MACKIE ONYXi series and the Presonus boards are excellent choices with a high return of value vs. cost. Particularly the ONYX boards. They sound great as a mid-level board and buying used, it is easy to retain nearly 80-90% of the value over a 2-3 year span. Upgrading is easy after you have matured your ears and learned the craft a bit.
I agree to buy the best quality gear you can afford while meeting your needs
. I also agree that a great deal of mid-level gear does sound average, but more common is the engineer using that gear has average skill or experience and produces average results.
It is rarely the deficiency of the gear itself.
Also, you will indeed want preamps (and that is a good thing to have available). The line-level signals (probably +4db) coming out of your beatboxes will not have enough gain to adequately drive a signal to any monitor source for practical purposes without some additional gain. The line-mixers mentioned back in the day had their own set of outs that fed into to 2 channels of the PA.
Either way, jump in. You will do great
. The most important thing is to get moving and enjoy every bit of the process. As Lao Tzu say's "A busy ant gets more done than a dozing bull..the journey of a 1000 miles begins with a single step". BTW, my very first mixer was a 16 channel Behringer 20 years ago. They were made in Germany and built to last back then. I used it to record my first (self-produced) album. Prior to that it was always in a 'big' studio. Album still sounds great.