The 1.9 tdi engine from what I know about it the internals are capeable of tollarateing a lot of power.
The probs u will have straight away is clutch will be eaten so a replacement updated clutch will have to be put into the buget. Also all boost pipes will need replacing so custom pipes will be in order but ull need a large front mount intercooler any way.
I would my self start with the basics to keep away from any problems
1. New full exhaust system( race cat or decat) followed by the rest of the system.
2. Intake system with not knowing the leon I can not
help u with what's best.
3. Large front mount intercooler with custom piping
4. Full custom remap on rollers with company who knows the pd engine well
5. Replace clutch with uprated clutch
Now I don't know what ur power would be at with these modds but wil be a lot higher than 105 bhp and if done in that order u should have no reliability issues at all
6. New bigger better injectors
7. Up rated fuel pump
8. Large Custom made exhaust manifold.
9. Bigger hybrid turbo.
10. New custom stage 2 plus remap
This is the order I would go bye
