I first got a benchtop DSO, then I got dirty cheap DSO154Pro to recklessly fool around low voltage, battery powered devices. That allowed me to learn faster about the basics than my fancy DSO with its thick manual and not-so-cheap price tag. Then I got into things connected to mains and got a cheap second-hand old analog oscilloscope for that. Should I be low on caffeine and blow it, well, that would be cheap, and they can do that pseudo-differential measurement mentioned by Conrad Hoffmann. I'm not aware of any cheap portable oscilloscope able to do that pseudo-differential measurement. Ideally you'd be using an isolation transformer to float the DUT before doing that.
However there are quite a bunch of threads about the controversy "isolation transformer vs differential probe". Main thing being, isolation transformer will nullify any protecting device in your electrical installation. No doubt others will give you better advice than me on that. Personally I decided to bit the bullet and get a Micsig relatively cheap differential probe. I could also get an isolation transformer in the future. You'll have to make your mind about this.
Only the "usual" probes are cheap. If you get, say, a 200 MHz rated oscilloscope and use 200 MHz rated probes with it, you'll be under 200 MHz bandwidth. To avoid that, you'll need, say, 300-350 Mhz probes, and that wouldn't be so cheap. There are OEMs selling probes rated for some bandwidth, but able to do more, so these would be good. Again, others here will be able to give better advice than me. But main point here is, probes are expensive, not just the differential ones, unless they are the usual run-of-the-mill ones.
If you need to look into decoding I2C or SPI, there are cheap -and not-so-cheap- devices that will allow you to do that. I got a DSLogic clone from AliExpress. A dirty cheap Salae clone didn't work for me, it turned out to be defective. Perhaps to be expected after that $8 price. Anyway, having a working logic analyzer, you wouldn't need an oscilloscope able to do any decoding, so an old, cheap, two-channels analog oscilloscope would be good enough to do all you want.
Problem with old analog scopes is "are they working good?". If not, usually you need an oscilloscope to repair an oscilloscope.
Last, since you are not the usual noob and have experience programming computers, perhaps you could run into some of these -rather rare- cases where you would really need a mixed-signal oscilloscope. The problem with these is, many times, when you use the digital channels, features get halved or even worse. Siglent's SDS1104X-E would be an example of that. But not the only one. You could do a search on this forum and find posts about it. You should look for owner advice if you are interested in any MSO. It wouldn't be an entry-level one, of course, if you want it to work OK.
Perhaps I'm not really helping, I fear, But it is what it is. You'd need to think about this and decide which way is your way, and go for it. Fortunately there's a lot of people here able to answer any questions you could ask, way better than me.
Good luck