It will need to be at least as large as the amount of used space on the existing HDD. So if its a 500G drive with 100G used, then a 240G SSD will be fine with the appropriate software that can dynamically resize the partitions while copying.
If you get something like the HyperX Savage retail box, it will have the adaptor and a license for Acronis True image 2014 or 2015 included with it. Juat install that on the existing setup first, then have it clone and resize.
Something like a 512GB Samsung SSD 2.5" and a 3.5" adaptor (or DIY adaptor from made pieces of aluminium if you wish).
Might as well use a 6G capable SATA III cable if your PC supports it. Disk mirroring software might or might not help as it could easily transfer all the failings of the present system to the new SSD.
Installing the OS from scratch on the SSD will result in much more sensible config defaults (otherwise you will end up with hard disk drivers with some ancient settings optimised for spinning rust).
Seek time is almost zero on an SSD so defragging just wears it out.
What CPU have you got and how much ram? CPU-Z will tell you.
On an i7-3770 apart from real time video editing I don't have any problems running 2D graphics and video playback. It only struggles rendering textures in games where you really do need a graphics card.
Before you do anything radical it might be worth running a few other benchmarks to see which if any subsystems are under performing. GeekBench and 3DMark are as good as any stress tests. (I expect others have their favourites)
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