[Hoovie]

I'll touch on a couple of things on your "wish list"

If solar voltage above X use solar,
When solar voltage below X use engine
When hook up voltage present use hook up

For the first two - If solar voltage above X use solar, When solar voltage below X use engine - an option you could use is the Redarc BCDC1240D B2B/MPPT (Engine/Solar) Charger.
This charger is a 40A unit and has a Solar Priority setup - so it will automatically use Solar as much as possible and will use the alternator to top up the difference between what the solar is providing and what the battery is asking for, up to a total of 40A
In truth, the benefit of doing this is quite slight and using either seperate B2B and Solar Controllers or a Combo B2B/Solar that works with only one input at any one time will work just fine.

Your next point - When hook up voltage present use hook up - I would suggest is not really the best way to have the setup.
It you are plugged in to mains, then you are very unlikely to be running the engine, so there is little point of working out the best way to manage that combo. That leaves you with Solar and Mains that can be potentially active at the same time. As Solar is "free" power (at least once installed) it is better to have Solar Priority where possible. The way I set that up where possible is to have the Solar Controller configured at a higher charge voltage than the mains charger. That way any topping up and trickle/float charging will come from the solar when that is available and from the mains when solar is not active.

Basically Mains and Engine are always going to be mutually exclusive in practice. Solar is potentially available when parked AND when driving and should be given priority over Mains or Engine. If using separate controllers, you can with a good one set up solar to always be the priority.
Use of an Arduino is possible but would requires a series of relays and control IO setups and would be a pointless (IMO) complexity for very little, if any, return.

 

[Hoovie]

doing stuff in stages due to budget makes sense but .... Do make sure you get all your cabling in place ready e.g. all the "first fix" stuff in house building terms.
For example, a common place to have a hookup plug in a T5 in under the bonnet. If you do this, then you will have a mains lead from Engine bay to cab. This will follow the exact same run as the Split-charge cable from Engine bay to cab, so it makes sense to run them at the same time.
You can add in the actual plugs and consumer units later on.

If you are a bit of a control freak you want to be able to see what is happening with the electrics probably (hence why you were thinking of the Aduino approach?). To scratch that particular itch you could (and are anyway) look at Victron kit which has probably the most advanced connectivity of the mainstream products?
As an example, I just logged onto my Campervan whilst sat at my desk in the house and took these two screenshots ...

A view of some of the devices (so Fridge is healthy at 3C, the Heaters fuel tank has used 1.436L since I last filled it up, the Charger is in Storage mode, etc)

And a summary of the charge - and energy production and use - of the system

I may be a bit of a control freak as well

 

[Hoovie]

If I have understood @Hoovie correctly, the MPPT will have two inputs, one from VSR and one from Solar. But otherwise i believe that circuit matches my intentions.

Not quite.
a Combo B2B/MPPT has dual inputs - one from Starter Battery(Alternator) and one from Solar - and uses one or other as per its setup.

The other thing I mentioned is where you setup your seperate MPPT Controller to be a priority over other charging methods. Doing that just means having the charging voltage at a higher level than the Mains charger or the B2B (or VSR if you go the relay route).
For example, my Mains charger is set at 14.1V, my MPPT controller is set at 14.2V (my particular batteries like between 14.1V and 14.4V for charging). So when there is a need for a little power it will come from solar harvesting, the MPPT will be putting in power first - as it is putting out 14.2V and the mains charger, seeing that voltage is fooled into thinking the batteries are fully charged. If the demand is greater than can be supplied from Solar, the battery voltage will tend to drop (thats how lead acid batteries work) and BOTH chargers will operate, but generally speaking, this setup means any maintenance charge comes from the FREE solar rather than the grid.