yeah slim towers (SFF - small form factors) are limited in hard drive space and graphics cards, they need a LP - low profile version, i have a gtx 750ti in a dell 7010 SFF, plays most games at pretty high details at a decent framerate.
Best to go for a full tower then you can fit a few hard drives, full size graphic cards, then if needed, a different PSU as well.
it is easy to build yourself, look on youtube for guides,plenty of computer forums as well for help and suggestions, if you want to go that route, a budget in mind it will make your decision easier
To give you some idea of costs, my GTX1060 6GB cost £239 back in oct 2016, now they are over £300.