See pic

The windows have begun to rust where they where put in so i want to take these ones out and fit new bigger ones.
I have found this company that supply windows.
http://www.leisurevehiclewindows.co.uk/
Has anyone used them before?
They do t25 windows which are 1067 x 429 which is approx an inch longer and wider than the current windows.
They also do t4 windows which are 1047 x 444
Do you think i would be better off going to the t4 one's as most of the rust is at the top and botton not really the sides.
Also how hard is it to fit new windows, is it something i can do myself or should i fork out for someone to do the job for me.
Thanks in advance
Matt