The biggest issues would be the small amount of RAM (256MB) and the 620 MHz CPU.
I mean, just look at this spec sheet: iPhone 3GS Full Specifications: However, actually running the phone might be more problematic on account of its minuscule CPU and memory. So long as you had a battery that had power left in it, the iPhone 3GS would turn on and work. Either way, it’s better than paying £2559 for one via eBay! Can You Even Use The iPhone 3GS In 2020? And apparently they all worked just fine. SK Telink tested all the dusty, old units too. How did it manage to get new iPhone 3GS models, when Apple hasn’t made any since 2012? Simple: it had a load laying around in its warehouse, so rather than selling them on eBay, it tapped the nostalgia vein of the masses and began selling off the old stock at a discounted rate.