I've been building specs for 40 years, it is not an easy way to make money. I do it as an adjunct to my custom business, I try to do 1/3 specs and 2/3 custom. I just sold a spec house for $4,350,000, my asking price was $4,400,000. The buyer started out by offering me $3,700,000, kept going up. I held firm because I knew it was a very nice house compared to others on the market. I am currently about to start 2 more.
As to financing, stay away from national banks, try local lenders. Plan on having to put at least 10%-20% of the sales price in as hard equity, if you are a new builder they might want more. Then plan on worst case scenario of having to hold the house and pay interest, property taxes, insurance, utilities, and maintenance for an extended period of time.
If you are new, plan on the crawl, walk, run plan. Start with one spec moderately priced, and build a program on that one spec. Use the best architect and design team you can find. Building specs is entirely 1000% completely different than building for clients.
Allan