Thanks for the help. I had to turn down the base costs to around 1/8 to barely scratch out a profit margin.
Like YNM said, just turn off inflation. Turn off city growth, too, unless you want huge cities.
I have started several games in 1700 with horse carts and sailing ships, and am usually able to get away with the absolute minimum loan of £10,000. Just find two towns with pop. > 1000 and a minimal distance between them, and set up a couple of 4-horse mail wagons. As soon as you can afford another wagon, buy it. Keep on until the service is saturated. Then repeat with the next couple of large towns. By 1710-1715 you should have a very healthy economy, and be able to move on to whatever cargo line you want to pursue. I'm in such a game now, and by 1733 I've got £800,000 in cash, and my company is worth more than a million £. I've got 352 2-horse and 4-horse wagons, and 22 brigs.
I don't have infrastructure costs turned on, but construction and maintenance costs are set at medium. Breakdowns are set to low. I like to play on a 512x512 map with medium sea level (makes for some nice shipping opportunities), hilly, and some variation. Number of towns and industries: normal.
Note that serving a primary resource with a high output, like a mine or a forest, is a distinct challenge with horse carts. Use 2-horse carts as they have optimal performance to length ratio, and use a separate road for each such service. You can handle up to about 100 tons a month on a dedicated road. Use 4-horse carts for everything else; they can move on the same roads as the mail wagons.
You should utilize ships as soon as you can afford it. Even the tiny brig does the job of more than 6 2-horse carts, and for a lot less in total cost. Within a 200-300 years' perspective, big canal projects give huge returns on investments.
A large horses & sails game doesn't carry nicely over to the modern era with automobiles. In a recent game, I had more than 4,000 carts in 1915 when the 10-ton truck arrived, and auto-upgraded all my horse carts. After a while it becomes boring to visit mine after mine to find endless queues of stalled trucks, and remove half of them. By 1930 when the 15-tonner arrived, I wasn't halfway through the station list. I finally abandoned the game around 1940. In hindsight, I should have enabled the HEQS grf and used the heavy trams for the mines and forests in the late 19th century - which I'm planning to do in my current game