Unless the OP lives in a hotbed of bankrupt upholstery shops, he's not going to find anything suitable for upholstery work anywhere near his $300 budget.
Most of the machines around here listed as commercial or industrial machines are relics from sewing plants, made for production sewing of clothing and useless for any sort of upholstery work.
I suggest looking at some youtube videos of compound feed walking foot sewing machines to see how a machine suitable for vinyl and leather works. There are some videos with slow motion showing the motions of the various components as the machine runs. Jamming a needle thru multiple layers of material is no big feat. The complexity and high cost of these machines is primarily related to what it takes to feed the material thru the machine at an even speed and make consistent length stitches. A homeowner type machine WILL NOT feed this stuff evenly regardless of any labels like "heavy duty" and regardless of how many fancy stitches it may have.
Dealers who have used compound feed walking foot machines of the type needed for vinyl or leather upholstery are typically asking 75% of new price for 30-40 year old machines.
I watched FB marketplace and CL regularly for months looking for any sort of reasonably priced suitable machine, and eventually gave up and bought a new Juki 1541S with a servo motor and table for about $1400 including shipping. When the used market wants a thousand bucks for 30 yr old clutch drive machines, buying new sorta becomes a no brainer.
Juki is the only machine still made in Japan as all the other former Japanese manufacturers have moved production to China. There are several Chinese machines on the market that can be bought for around $1000 including shipping. I don't know anything about the quality of any of them, and spent the additional money to buy the Juki because it consistently has top ratings for general purpose vinyl and leather use.
Now, if I could just find the time between other projects and honey-do's to actually learn to use the damn thing