Muzzy,
That brew enhancer; has it got any maltodextrin in it?
If so, I would use it, along with the 1kg Morgans Caramalt - you can always add some clean water to the fermenter to achieve an original gravity of around 1.050 anyway. I like my Bavarian Lagers to have a little bit of body in them - I find that a final gravity of around 1.012 is nice and gives the beer a malty lager feel - which I reckon is appropriate for a ballsy German Lager...but that's just my opinion!
Good to see you're going to use the right hops (Hallertau) for the beer - are you using pellets, flowers or a tea bag? If you use pellets or flowers, do a small boil (approx 10L for 20mins) and toss in the can of Caramalt - that way you get to key in the hop bittering and flavouring resins and oils with the malt and end up with a better brew - you can add the brew enhancer sugars and the Bav Lager kit at flameout and stir through gently to mix the enhancer into the concentrated boil. Just watch the boil when you add the hops - you can get a bit of froth, so make sure you use a decent sized pot for the boil with plenty of headspace!
Considering its only a 20 minute boil and you are using the hopped extract kit (which should not be boiled - just add it at flameout so you don't boil out any of the flavour and aroma notes), I normally use around 25g of 4.5% AAU Hallertau pellets for this beer.
If the brew enhancer is just dextrose, then all you're doing is upping the gravity and resultant alcohol without improving the body or flavour of the beer.
It's your beer - and your call!
Cheers,
TL