My now switched off (XP) spare would not run either malwarebytes or defender whilst I was, say, on the net and/or doing mail and/or using Office
I would hazard a guess that it was a single-core CPU.
A dual-core or better CPU > 2GHz with 4GB RAM and a moderately good graphics card with > 512MB VRAM and a HDD of 7200 RPM should suffice for the limited usage you envisage.
I bought such a PC from ebuyer for £199 a couple of years ago for a relative, and it's still going strong. Without monitor, keyboard & mouse, but you probably have those already. You can always upgrade RAM / HDD / graphics at a later date if you want / need to.
I prefer self-building, as I can select exactly what I want, but I can understand your reluctance to do so.
SD