Saw a tv programme about 6 months ago. About 12000 army recruits, half measured and given best shoe for gait etc and other half given any pair that fit correctly. Six months later the injuries assessed and there was no difference in number and nature of foot/leg injuries. They suggest fit is the most important factor so you could be right on getting budget shoes.
Dont forget, all your training is done now, do not do anything silly this week, just go for a few simple little runs,a long run now would ruin you for next Sunday, nerves and temptation will make you want to do a long run, dont give in. Good luck for the big day, you have loads of support from this site alone.