I understand you being careful with regards to the different liveries that are out there. I think that small changes such as the lettering used for Regional Railways, Arriva, is a small change but its big enough when it affects the time era for the model in such a way. I would think that standard regional railways engines are the easiest to go for, as I dont think people would specifically model the area in a deliberate time period for Arriva Trains Northern opperation, when its just one TOC running at the time. The liveries of other units youd buy would overlap and so I dont think people would be that fussy generally.
With regards the two ideas for a northern 143. Well... these are the earliest liveries to do, and so I guess would be people returning to the hobby or those wanting to specifically model that area and time. Id fall into the second group. The provincial and Tyne and Wear 143s were allocated to the North East branches such as the Tyne Valley, Durham Coast, Bishop to Saltburn, Darlington/Boro to Whitby and Newcastle to Morpeth/Alnwick. That makes scope of opperation all the more restricted as they arent exactly top billing for layouts of the time - but Ive always held the view if the companies make them then people model the areas the trains run. The Tyne and Wear one would be select but a great one to do - the added bonus is that if Regional and Arriva are two seperate liveries then you can have two for Tyne and wear as they did change the Tyne PTE logo when they were done. The provincial one would be perhaps the most smartest 143 to do but both would be a while as more pressing and recent units need to be done to get sales in for your company.
So that leaves a 144 - and with a 3 car to come? ... Id add my name to that list as Id love one of them too.
The model looks fantastic and is setting the standard by which all other new DMUs should be made.