Please need advice , serious matter!!

[motoadve]

Bought my 2005 Boxster 2 days ago, Im super happy with the car.
The problem is I have found out the guy who sold the car to me its a bit shady and not to be trusted and has obscure bussineses.
The car is clean and original, but he only gave me 2 keys and not the plastic key, first he said he had it , then when I payed he said he lost the key, then I called asking about the key and he says that he never had it.

So now what are my options?
Maybe he really lost it and at the end tells me he never had it so I stop bugging him.If he was a regular guy, no problem I leave it there, but not after I heard about how's this guy.
Maybe he has it and .....?
So I was thinking on adding a gps tracking system or an immobilizer system to add security to the car.
This systems will void the warranty though, and I have 9 months left.

Are those systems reliable or they can make a real mess of the car computer?
Any other simpler security methods?

This is in Costa Rica by the way where the insurnace will only give you 30% of the car value.

Changing the ignition key and door locks is another option but will problably cost a fortune.

SO what Im looking for is to add a security system as simple as possible that wont mess up the car.

Advice is welcomed , I couldnt sleep with this potential problem.