It’s not so much a Let It Ride thing as just a “What is going on in the fiction?” thing. Once you roll Resources to have a thing, you have that thing until you lose it for some reason in the story, such as a consequence for a failed test.
So you only need to roll Resources for your horse once, when you buy it (/ want your estate to already have it). Afterward it’s part of your lifestyle maintenance. If you consistently fail lifestyle maintenance, one thing that can happen is your start to lose your stuff – you end up having to sell it to make ends meet or your stuff goes bad because you can’t afford its upkeep.
It’s a matter of taste, but I wouldn’t allow a character to receive Resources. A title and keep is much more likely to be a Fund, maybe 1D or 2D. Then it’s up to the character to maintain his new standard or go broke trying. Estate (mis)management is a big deal!
An NPC can give the gift of a horse and carriage and driver or whatever else, of course, but if the gift is an estate I’d treat it the same as any property purchase from chargen. It might come with those things, but it’s up to you and your Resources to see whether you can maintain them and keep them or whether it’s going to drive you to penury.
