Hey Sikon,
Would definitely get an ssd, the difference is night and day. You could always get it and then reuse it in any future custom build anyway. You could go the cheaper route and get a smaller one and put the operating system and any frequently used files/programs on it like Cod and keep the current hard drive for extra storage, or go with a larger one and just put it all on there, they are fairly cheap now, this is what I went for a few years ago. You get the added bonus that the computer is really quiet as you don't have the old mechanical hard drive noises. When I switched mine out a few years ago all I did was clone the old hard drive onto the new one, didn't have to reinstall anything.
Your computer will take a traditional ssd with a sata interface, here is an example of upgrades for "Aspire AT3-710-UR56" on the Crucial site : https://www.crucial.com/compatible-upgrade-for/acer/aspire-t3-710-ur56
It just looks like you can't get the newer m2 versions but rest assured traditional hard drive to normal ssd is still a huge difference.