I think the idea of buying Pro Evo 4 from play for £9.99 (delivered) is a good idea, if you can be fussed getting it online im sure you will be able to pick it up second hand for a tenner or there abouts. From what it sounds, Pro Evo 5 is going to be definately more of a game that Pro Evo 4 but also a little bit more complicated if you dont have a pro evo back ground. so for a small chunk of change grab Pro Evo 4, play it to death, see if you like it and if you do, move on to Pro Evo 5. that way there is a lot less risk of blowing £30 on a game you wont enjoy.
That said ive been playing pro evo/iss since it was released back on the old Nintendos and never felt the need to touch fifa since i got a free copy of Road to World Cup back in 97, as have many people from what i gather from the posts on here, so there shouldnt be to much risk of you not enjoying it.