I can fully understand your line of thinking Matt, if I only had one PC I would be wary about risking wrecking it. Your current HP550 looks pretty high spec to me (or am I completely behind the times?) so buying something similar isn't going to be "dead cheap", but I would certainly explore the refurbished/second hand market.
As for which Linux distro, Ubuntu and Linux Mint are both popular, and I think Mint is a bit lighter weight and uses less resources, although Ubuntu wouldn't have a problem on a machine with that spec. Somebody once wrote an article listing a number of reasons why Mint was better than Ubuntu for complete beginners, but unfortunately I can't remember where it was. If you do a search on something like "Ubuntu vs Mint" you may find it.
Per what Brian has said, download a few distros, burn them to USB sticks and try them out. This way you're not committing to anything. On Windows I personally use Rufus for creating the USB's, but there are other options.
I'm currently running MX Linux on one machine and Manjaro on another, and I haven't had any significant problems with either. I always say that if I can figure them out then anyone can.
The other thing to remember is don't give up at the first hurdle. If you come across a problem then ask either here or on the forum for the particular distro in question, there's always somebody willing to help. Some people have issues just creating the USB in the first place, so don't worry about it.
Please let us know how you get on.
As for which Linux distro, Ubuntu and Linux Mint are both popular, and I think Mint is a bit lighter weight and uses less resources, although Ubuntu wouldn't have a problem on a machine with that spec. Somebody once wrote an article listing a number of reasons why Mint was better than Ubuntu for complete beginners, but unfortunately I can't remember where it was. If you do a search on something like "Ubuntu vs Mint" you may find it.
Per what Brian has said, download a few distros, burn them to USB sticks and try them out. This way you're not committing to anything. On Windows I personally use Rufus for creating the USB's, but there are other options.
I'm currently running MX Linux on one machine and Manjaro on another, and I haven't had any significant problems with either. I always say that if I can figure them out then anyone can.
The other thing to remember is don't give up at the first hurdle. If you come across a problem then ask either here or on the forum for the particular distro in question, there's always somebody willing to help. Some people have issues just creating the USB in the first place, so don't worry about it.
Please let us know how you get on.