You certainly could turn the club into a sustainable Championship team with that kind of money though (well you could before sisu blew it)
Buy the stadium, put the facilities in place and upgrade to a tier 1 academy, build a pub, hire a decent marketing team and give them a budget, decent club shop, properly staffed and stocked, install a new ticket system, enough staff so when you call you get through. all for under £15mill
Scrap the SCG, start a forum where all season ticket holders can post, this would be well moderated, allow constructive criticism but no abuse, allow fans to vote on things that are important to the experience, have competitions to design things like banners, marketing posters, things to make people feel involved. Make up the best banners nice and large they will be used at every game.
Fund a cheap season ticket programme for 2 years. Everyone who has had a season ticket ever at Coventry City, or has been to 20 home games over the last 15 years can buy 2 season tickets for £100 each per year, any parents that take this up can get a free child season ticket per adult.
Now you have a large amount of people at the games and 2 years to win thier love back, put alot of thought and effort into the experience.
Work out expected turn over in year 3 and build a team around that (we are accepting 2 loss years here to build the fanbase back up)
2 year plan for promotion and fund this accordingly, along with a talented manager who likes working with youth.
The team will eventually consist of players who have come through the academy along with a few very good and experienced players. No longer sell academy youngsters at the first opportunity.
After 2 years you are in the championship with 20k+ fans who love the club, I think you could do all this for 25 mill and at the end of it you'd have increased the value of the club by more than 25 mill.