The analyzers we use are very similar to the type used in hospitals to keep patients alive...they are accurate in the sense that consumer GPS is accurate. Sure, military GPS might be more accurate, but consumer GPS is more than enough for day to day. Siemens guy is nerdsplaining, don't pay too much attention to him.
You can see the very minor differences from temperature, humidity and gas pressure/flow yourself, by testing at room temp, storing stuff in cold (fridge, ice water for tank), then storing in heat (car in middle of day for a tiny bit), and with valve open a lot or just a little. It will be small, it won't turn your 32% into 80%.
I would look at the design and size. The home built one will be bigger, bulkier, and possibly more fragile than say, a Palm. But the nicest analyzer I ever owned was a homebuilt one that was built into a pelican case, with an aluminum sheet custom bent and milled out, it was awesome, but big and bulky compared to a tiny handheld unit that I can throw into a case. Traveling on a plane would have been impractical, but for use around home, it was great. I should not have sold it.
I don't want a screw on DIN fitting, because sometimes you have lots of tanks to confirm. A standard rebreather dive has me checking 5 tanks, three of which the regulators only come off for fills, and I don't fill up each dive. I check those with the half ball that you shove into a second stage mouthpiece, or into a valve opening, or with an adaptor (QC fitting screwed into a 1/2" PVC end cap) into an inflator hose.
What is important is to leave room in your budget to also buy a CO analyzer. CO will kill you just like having the wrong o2 percentage.