While I respect your opinion and decision to build your own grinder, time and experience has proven to me that the vast majority of "beginners" possess neither the tools or ability/knowledge to build a grinder that is even remotely as precision as one of the "top end" grinders. Yes, they are expensive, but when you pay those "big bucks" for a grinder, what you're paying for is the precision, durability, and longevity.
I have a student in the shop right now who first tried to build his own grinder, ironically, out of square tubing, and in his words, "it was a nightmare". Then he purchased a "cheap" grinder because of all the issues with his self built one, and after one day on the KMG in my shop, he says he is going home, selling/getting rid of both, and ordering a KMG. He also purchased a Teco brand VFD and motor, because "it was the cheapest I could find". The VFD failed within a week, and he's having issues with the motor too. Overall he says he's spent approx. $2k between trying to build his own grinder, scraping it, and then buying the "cheap" grinder, Teco VFD, and motor....... and in his words..... "Live and learn".
This is a scenario I've seen time and time again..... in the name of "saving money", individuals spend endless hours building and trying to make something work, that simply won't, and by the time they relent, they've spend enough money to have purchase 2-3 of the "top end" grinders. I believe that's what's called "gaining experience".
Are there people out there who have the ability to build their own grinder, AND produce a good product using it? Yes, but typically those people have had years of experience, in a number of varied areas, that have given them the skills to do so. They are also fairly rare. All that being said, I would say by all means, if you honestly believe you possess those characteristics, and feel confident doing so, then of course, build/use your own grinder(s), but for those with lesser fabricating skills/experience, who are doing it solely because they feel they can't afford a grinder, it usually turns out to be a "live and learn" situation.
I'm certainly not saying that a person MUST purchase one of the "top end" grinders to be successful, but after 30+ years of doing this, I can say that a person is money and time ahead, and the learning curve is much shorter, if a person makes the investment in a "top end" grinder. The other side of the coin is resale value.....build a grinder, and that money is gone. Buy a top end grinder, and in a decade, if you decide you want to sell it, you will get nearly the same money as you paid for it.
I can only offer advice based on a long knifemaking career. In the end, each individual must evaluate their own situation, based on endless factors, and make the decision(s) that are best for them.