Welcome to indietalk, Rob!
Budget:
No way to know what sort of budget you should be looking at
without reading the script. 5 minute shorts have been made
for £50 and for £50,000. You will need to do a complete break
down.
Funding:
Since in this case you are asking people to invest because they
believe in you and want to help - not for financial return - you
have to make this very personal. Don’t make promises about getting
into festivals - though do you research and tell them about the
festivals you plan to enter and how much that’s going to cost.
However, plan a showing of the final movie in a public place so
everyone can see it.
Do you research on what TV stations show short films and
include that info in you "business plan". In this case you are selling
yourself as as an up and coming, dedicated movie maker.
Attaching a name:
If you can, it will help. Do your research. Find out how many "names"
attach themselves to five minute shorts and then find out how many
of those actors attached themselves to a project in which they didn't
know the director personally.
Since you will be using this actor to get financing, they become
primary partners in the production. Without them, you can't get
the money. So you aren't actually making an offer, you are asking
for their help. You will use their name to get financing.
You have to remember, any actor worth having attached to your
script is already getting a stack of scripts from producers and
studios. So what sets your project above all the rest? They will
ask - so you should be ready with an answer. Think about how
doing this movie will help them, not you.