Good advice given so far.
All said by SIZ is correct and MV gave a very good alternative suggestion.
The optical adapter that is made in China has very cheap glass, distoring your IQ and adding significant CA. This is to say it must only be used with Tele lenses. When used with wide angle or even 50mm, its inheritant magnification of x0.25 factor, totally rouins the wider angle of FOV that you need. Canon on very early days of introducing EOS lenses i.e. EF, made a quality adapter but first of all you are lucky to see one for purchase, then even if you find one, it would be about $2000 or more. So I really don't think you want to save on cheap old lens and go wild on adapter.
In short, if you really need quality manual lens at low price, look Rokinon and several other rebranded name of same exact lens such as Samyang, Bowel etc.
If you are to get old lenses, Avoid Canon's older lenses (FD or FL) also Minolta M. These two need optical correction due to same longer focus point you mentioned but Nikon, Pentax, Olys or many other lenses including all 42mm screw lenses are fine and can be use on EOS only with a physical adaptor that needs no distoring glass. But then the key thing is that old lenses are made and tuned for Film. You will see the ugly color tune and shift of color due to very different lens coating and then due to lack of computer assited lens design and manufacturing, most cheaper old lenses do not have Aspherical component thus further degredating your final image.
I love the old lenses but mostly to see them in my collection not for use.
Hope this helped.