I have bought and refurbished many breville baristas and infusers on Craigslist. I've bought express as low at $250, but typically a good price is $350 used. Granted I'm outside NYC so they appear more often than other areas.

I was happy with it, but the grinder was subpar, and not having the 58mm basket really limits you. I went from that to a refurb rancilio Silvia and it was so much better.

If you do go with the 54mm portafilter, I recommend buying the distribution tool from Crema. It's high quality and really helps with a subpar grinder. https://www.cremacoffeepro.com/

I paired my barista with a sette 270 for a while and it was a world of difference but my rancilio outperformed it In every aspects.

Pancrake's comments highlight how, when looking at entry-level gear, there are significant gains possible with "small" changes in budget. This is especially true in the low-budget range. Even at the same budget, careful choices can make worlds of difference in both learning the craft, as well as the typical results you potentially can get.

For what it's worth, I've been using an Infuser for the last 2 years quite happily. It's not perfect, but it does make good to excellent espresso once you get to be familiar with it. The quality of grinder is, admittedly, crucial and my Niche has paired with it quite well. Perhaps you'll grow out of it and want something nicer; I just upgraded to a DE1+ for the obvious benefits, but I've never had a problem pulling consistently good shots with the Infuser.

With a little patience you can get a refurbished one with a factory warranty. I think I got mine for $300 with a warranty and haven't had any problems with it since the day I bought it. Sure it's far from a panacea of espresso making, but it gets the job done for a fair price. (I also would not buy it new for $600, though.)