I've decided to soldier on with trying to convert the Bachmann chassis. Several reasons...
1) The Ultrascale conversion pack wheels (well, the axles and crank pins to be correct) are designed specifically for the RTR model so I'd have to buy new ones - not something I'm about to do given that I spent over £100 and waited almost a year for delivery of the four conversion packs I now have in my possession!
2) Converting the Bachmann chassis, even if it does involve some fairly major surgery, will challenge me to go beyond my modelling 'comfort zone' and improve my skills. After all, if I really want to progress beyond 'out-of-the-box' modelling into true 'finescale' then a bit of RTR butchery is a good stepping stone. And even if I make a complete bodge of it I will hopefully have learned something in the process!
3) My decision to 'go P4' was primarily based on a desire for accuracy of appearance, something which is still important to me. Even if it's possible to achieve a good looking model in '00', I know that I'd eventually get to the point where I wished I'd just gone that one step further.
4) I do tend to have these moments of panic occasionally, but in the vast majority of cases the situation is never actually as bad as I first think it is!
5) As I've said before, I believe strongly that the finescale 'ethos' needs to be pushed more through easy access to information, resources etc. I'm going to try and keep track of the steps I go through in the conversion and photograph things as I go along so that if and when I finish it I can post a step-by-step guide that might be of use to others.
So there you go, crisis of faith over... for now!