Kevin
With Diagnostic Units it is a case of you get what you pay for.
A cheaper one will diagnose most engine faults/codes, but for other systems you usually need a slightly higher priced unit.
I have two, a Foseal cheaper model with is around £20.00 and copes very well with engine needs. Then I have a iCarsoft CR PRO , which covers all makes of car and which is very good for all systems and is around £225. There are cheaper models that have the software but are specific to car make. I.E. Jaguar.
To my mind a diagnostic unit is an essential part of owning and successfully being able to run a Jaguar. That is of course my opinion, which others would be very entitled to disagree with.
Good Luck with sorting your car.
Best Wishes and Regards, John