I have a old 2007 Mac Mini lingering in my office – I think that's beyond any reasonable use other than a plant stand. It's not like I need another NAS as the other Mac Mini is doing that. And there's a G4 Powermac tower in the summer house, which I liberated (ok, I just forgot to mention it when they were clearing the office) when I was made redundant from a previous job. And I charged the taxi I used to liberate it back to work too. Ha. And it turned out that they had to rehire me at consultant rates and, as they'd misplaced that Mac, get me a newer one. Which I, at least, didn't steal. Though I really should have.
I fired it up the other year to send a fax. Who knew I'd need a modem in 2016. I think it took an entire afternoon. Anyway, still looks cool and is an amazing piece of design, but it seriously weighs a tonne (have you seen the bloody heatsink) and the fan, oh my, it's like standing next to an idling 747. Computers were a lot noisier back then.
Anyway, I'm still impressed with the capability and speed of my old 2009 Mac Mini. That will only run MacOS El Capitan, later Mac Minis will run High Sierra. The forthcoming MacOS Mojave will only support 2012 Mac Minis and upwards. And my poor 2011 MacBook Air which is still going strong (other than the battery, which has developed charge-based incontinence).
Anyway, check compatibility before upgrade. There are perks to upgrading, the recent MacOS releases have been solid and have delivered significant performance improvements especially for SSD-based machines. Do buy an external disk and run Time Machine on it. Even SSDs fail, and it's the minimum backup you should have (ideally consider some cloud-based storage, even if it's only vital files to Dropbox).
I'll add my usual plug for Affinity Photo, because I really like it, and I think it's a more than able replacement for Photoshop in most circumstances at a very reasonable price point. There is, of course, a learning curve. Also, if you're wedded to PS, Adobe still sell Elements, which is basically a cut-down version of PS but with some familiar interface elements. But it sounds like CS5 will work for now.