The higher your tech-level is, the more infrastructure - both physical and social - you'll need to maintain and operate it and the more diverse raw materials you'll need.
I'll grant you that point however you have that on the homeworld. If the colony is expected to survive then I expect it to be supported. This means not occasional visits by a Free Trader who wanders past but a regular shipping transport dedicated to the colony. Even if you only want to support the occasional Free Trader (Note the capitals refering to TNE's Free Traders as a people, not free traders as a starship class) you will still want some form of starport, and that will require your TL9+ homeworlds tech. For example, look at the other mission in World Tamers, it has a number of Clipper modules all built at TL9+ sitting in orbit acting as a command post, starport, medical center and survey point not to mention a storage point for supplies. At no point was this considered either too expensive for the mission, nor were there any complaints that the locals at TL-2 couldn't repair it.
Golan hit the nail on the head, it's infrastructure that counts, and that is built over time. Also, the more complex the infrastructure, the larger and more specialized the population you need to support it. My colonization scenario in World Tamers supposes a group of people with a variety of skills signing on to build a society on a raw, unexplored world, not dissimilar to the colonization of the Americas in the 15th and 16th centuries.
I was wondering when this old chesnut would come up. Oh the glorius whimsical pridefull memories of our nations historical foundings and early development. So easily we forget that our forefathers came from a technological society, atleast in Travellers terms, of EXACTLY the same Tech level. Those muskets and plows that built our nations were of the same Tech Levels as those used by our homeworld equivelent nations of Europe; England, France, Spain, Portugal, the Netherlands et al. The English colonists did not establish a colony in the America's at Stoneage TL-0 because it would be too expensive to import flints for their High tech flintlock muskets. How would we ever replace a plow blade should we break one? I know, we'll make do with bones tied to sticks as plows because that way we'll atleast be independent. Sounds silly when I put it that way doesn't it yet people cling to this romantic idea of building from scratch just to be independent and self sufficient.
While it is possible to import some high-tech items from off-world, these will be rare and expensive. It will be decades before an industrial base is established that can handle the production of TL-4+ items. If the colony world has a valuable resource in demand off-world, then it can earn cash through exports that can be used to speed up this process. If not, it will remain an agrarian society until they develop industrial and transportation infrastructure.
This point is probably where I found the most problems. If a colony world has a valuable commodity for exploit then it will be exploited, and then some, and with the most efficient means available to the Homeworld concern that is doing the exploiting. Infact this will be the reason the colony is there in the first instance and they will not care a whit if it's agrarian capabilities are up to snuff. In addition, all it's high paid workers will expect all the creature comforts they have back home on their TL9+ homeworld. Look at any Western, First world, minning sites built in Third world nations for examples. Heck take a look at US forces in Iraq with their own McDonalds stores!
If there is not a commodity or resource begging to be exploit then the colony in question is allready on shakey ground and is looking more like a dumping ground for convicts and political dissidents. I know that even this type of colony can succeed as I sit in my living room right now in a former one. Though I would claim that a, it was supported, b, there were/are resources to be exploited, and c, that it was established at, or certainly with, the same Tech level as the English that created it were using.
About horse, donkeys, mules, oxen, cows, and other farm livestock.: I never did say they were cheap to maintain. What I said was they reproduce themselves free of charge. You can bring in the first generation of livestock to the colony world either in special low berths or as frozen embryos that can be thawed, raised, and then made to reproduce and used for transportation and food. You may also find animals native to the colony world that can be tamed and or eaten. Either case would be cheaper than trying to maintain a hover craft or air raft for any length of time on a colony world. You need spare parts, skilled mechanics, and fuel for that.
Anything is possible on a colony world including a mix of technologies. In the Epilog to A Long Way Home, you see a picture of a colony that has a steamship, sailing ships, blacksmiths working metal, one or two imported air rafts, and a hand-full of ground cars. But mostly our colonist rely on horse power to get around. By the time I write the sequel to this book, I will have developed a more comprehensive picture of this colony world.
I haven't made that purchase, yet, so I'm wondering what exactly is that colonies purpose as that will have the strongest bearing on it in my opinion.