Cuellar enjoyed an impressive season at Ibrox in 2007/8 before making the move to England to join Villa.
The 29-year-old defender fell down the pecking order last season and Rangers have been holding talks over a possible transfer since identifying him as a top target earlier in the summer.
A significant breakthrough has now been made in those discussions with the two clubs agreeing a fee, and McCoist is hopeful of re-signing the Spanish defender.
"That would be great but there is still a bit of work to be done," he told Sky Sports News.
"A fee has been agreed between the clubs. In an ideal world, we would get the deal over the line and get Carlos up to rejoin us because he was a wonderful player for us and was thought of very highly.
"We have made no secret of the fact we want him back, so hopefully Carlos and one or two others will join us in the next week."
McCoist is keen to bring in a number of other reinforcements before Gers start their SPL title defence with a home game against Hearts next Saturday.
Bursaspor striker Kenny Miller is one player on their radar but McCoist does not know if he will come back to the Glasgow club for a third stint.
"That is a possibility. We haven't been in touch with Bursaspor at all but I wouldn't rule anybody out at this stage," said McCoist.
"We are certainly looking to bring three or four players into the squad for the start next Saturday."