I would really recommend offering a better rate for the work - most artists expect between £10 and £30 per page so offering just £50 for a whole stack of pages is asking for a lot.
A page can take 2-3 hours to create. That means 18 pages easily takes 36+ hours. Would you work a job that paid you for a full week's work with only £50? That's paying less than £2 an hour.
Obviously you're not meaning to put people off or make them feel put out, but I'm just trying to give you a bit of perspective. Some of us do commissions to pay the bills so we would want twice the amount you're offering at the very least.