Ssd would only help load os and game faster but no actual gaming performance boost. you can use a psu upgrade installed outside case although not elegant.  You can also run entry level gpu such as gt730 or 1030 which will work with 220w stock psu. it is better if you move components except psu to minitower case which can be inexpensive.  But if you are inexperienced in pc building it may be a somewhat challenging project.  On the other hand you will learn from such endeavor and gain experience. here is spec of stock psu  
I'm looking to upgrade an old office computer into a gaming computer. I have very little experience with computers, but this is what I have found so far. The case it is currently in is too small for a proper PSU upgrade if I want to add a GPU(I currently have the Intel(R) HD Graphics 4600). It already has had a CPU upgrade to an i5 4460S and 8gb of ram(2x4gb). I've also updated the BIOS to the latest version but I'm struggling to find out what to do next. It has the original motherboard(02yrk5) and PSU(220w). And should I add any SSDs? It only has its HDD(st1000dm003-1ch162). Any information that people have would be really helpful. Thanks
Upgrading a Dell inspiron 3647 Case, psu, and gpu