That of course brings up another option, the first "High Port" a planet might put in place an old ship (or ships) that is up for scrapping due to being not worth while to replace a failing J-Drive. A minor planet might still find it useful especially if they add a cargo airlock once it is in place and an UNREP to fuel the incoming vessels (using the original fuel tanks of the "station as fuel storage). They of course would also need one or more "shuttles". At that point unstreamlined vessels can visit the planet possibly increasing competition and more ships coming in.
This works especially well for MTU where modular ships are common in many areas. Get an obsolete 1-2000 ton ship with several standard cargo modules (30, 50, 100 and 500 tons sizes are common for smaller ships) and an incoming ship ideally docks at an empty module port and unloads its module and then moves to another module port and loads a replacement, refueled via UNREP from the ships own fuel tank (on a fuel module) and is quickly on its way. Modules can be anything you like, cargo, passenger, science, hospital or prison for a few examples.
Again you would need appropriate shuttles to take things down to the planet (or elsewhere in the system). When you build a "real" High Port this "station" could be moved elsewhere in the system. An advantage of these is that you can modify existing deck plans if using non modular ships.
The books don't lend a lot of layered sophistication to orbital infrastructure. Grav technology fundamentally changes everything about space access. A busy planet, especially one that combines a large, industrialized and high-tech population, is going to have a massive space infrastructure. Add in one that sits smack dab on one or more major trade routes and you will find an even higher level of industrial, warehousing and habitation platforms.
In cases like that, 'highport' is going to be terribly useless as a description. There won't be a singular port, there will be dozens, if not hundreds. It makes much more sense to put your major port (for cargo at least) between 95-99 diameters away from the planet to minimize travel time for ships that don't need to interface with the planet. Hub-and-spoke passenger traffic builds upon having passengers at a centralized location, so they'd be proceeding to the station(s) orbiting the planet. Cargo, which is much less perishable and may get stored for longer periods, can easily be stored in orbital warehousing waiting for the next ship to pick it up.
As far as UNREP goes... that's really not something you should be seeing at a station. UNREP, by it's nature, is meant to deal with ships underway rather than docked. A station would have far more robust infrastructure present to handle atmospheric, waste and fuel interfaces that ships would want to deal with while docked. It would be very similar equipment, but not UNREP.
As far as modularization goes, I think that's gonna be a toss-up. There are advantages to it, but a lot exist on paper and don't take into account the disadvantages and drawbacks. Lot's of great ideas have been proposed but, for various reasons, fail when brought into the real world. Things like fast air transport, fast sea transport, LASH shipping, etc. All had positive points on paper, but the harsh economics of reality doomed each concept because of costs. Merchants are cheap and they will always seek out the lowest costs possible to perform basic services.
