Solid proof comes from actual customers, clients. Greg, from an investors prespective there is so much less risk investing in something that is already making money even if its a little bit. If you can sell it once, and investor knows with his / her money, you can increase your sales , R&D, support, marketing 10 fold, 100fold for 100k fold.
I would strongly suggest bootstrapping. A LOI is nice if you can get it, but most clients wont agree to sign on. LOI are more used for larger projects where you have to get many players to play ball to make a deal happen.
If its something you can do solo, with your own money, family | friend money, go that route first. Be proud to tell investors, you dont want their money. And only borrow as a last resort, when you have to, or when the investors bring substantial effects to your business.
My approach, dont sell the investor on what you could do for them, ask them how they could help your company grow and what is their expectations in $$$ gain for it.