>>106948755
>Which DELL Optiplex should [I] get
Ideally you want one with a 7th gen i7 or better (i.e. ≥4c8t). Try to get one with standard ATX parts but more recent models will most likely be proprietary.

>and which low profile GPU should I put in it?
The best card that runs off slot power alone is the 3050 6GB but do be aware that some models require supplemental power.
If your PSU happens to have PCIe power you can go up to a 5050 or maybe 5060 (or used 4060, 6600, etc.). Not much point going higher because a new PSU will eat up all your savings.
The 1650 non-super is worth a mention if you can't afford the other options.