Hi and welcome
Recommending a remap is a little bit like recommending a wine to a friend - you may love it but your friend may not! It also depends on where you live and how far you are willing to travel.
The GTTDI is a relatively easy engine to remap compared with its petrol equivalent and most tuners will be able to give you what you want. I have used Superchips in the past for the Diesels I have had and none have given me any problems. There are other tuners on the market (GIAC, APR, REVO etc) who will remap your car through the OBDII port or you could go for something like a TUNIT module which fits between the ECU and the engine. I have heard of Upsolute, but not had anyone posting on here who have given any comments.
With regards to an Alarm/immobiliser - Some of the tuners (APR for example) can have an immobiliser function added on to them.
If you are after a separate system then I have used Clifford in the past and always found them okay (if sometimes a little over-sensitive)
Huff 18"s - never tried to buy or sell them.
