The Intec is OKI based, completely made around OKI engine, if you peel off the labels and put two next to each other - you won't guess which one is which.
It is sad that I have to explain this to you for a second round - guess it is not what you want to hear - sorry.
There is NO over 300gsm spec machine that is small-ish
No Intec will not do you any good - the way they achieve the higher GSM is by adding piggy-bag electronics and some programming to make the machine go slower. OKI, it is based on, is 36 page per minute on 20gsm bond paper, can you imagine how slow it would have to go to run 350gsm print? Find a way to see it in action printing 350GSM+. Want to print 30-50K per month on it? Well, you'd need to have your days stretched to be 36hrs not 24

Besides INtec sells OKI supplies at about x2 the cost of those, generally same OKI supplies. so do figure out your per page cost.
Keep in mind - no, you will not get 15000 out of toner cartridge, thats at 5% coverage which is nothing in our industry, realistically 5000-7000 pages and with the big IF
-no you will not get nice consistent 30,000 pages out of DRUM
-fuser 50,000 running cardstock - not likely, probably half
-Belt 50,000? I'd out that.
If you fall for it anyway, contact me - I will help you using all ORINAL GENIUNE SUPPLLIES for, probably, half what INTEC wants.
Did you say 30-50K per month? thats half a million per year, how strong do you imagine these mid-size printers are? and once again, are you ok with 10-15 cents per page cost?
I was right - KM Bizhnub C7000/8000 can go over 300gsm so that is the only choice (sure there are similar duty/spec machines form other manufacturers) according to what you're asking. You can not have both be on budget and have 350gsm printer. That is unless XANTE or INTEC will be called in to the game but once again - look up supplies prices for OKI CX-3641 VS XANTE VS INTEC. Once again- consider that with 350GSM media they will push 10-15 pages per minute on 8.5x11 / A4, half of that or SRA3 - how are you planning to run 30-50k impressions per month, I, really don't know. Run 24/7? But then again, these printers are not designed to run 24/7 even a glorified OKIs like Intec and XANTE.
My point is that you need to rethink your business model and probably the budget according to reality.