Hello
You got experience with a language that can interact with SAP, so you can begin by this part, and then learn the ABAP as part of your work. You will however need to learn ABAP, as you can not always rely on an ABAPper to explain you why/how somme data are calculated/retrieved on the ERP. Also being able to code you own functions is useful (still, pay attention to the company ABAP coding standard :) )
I'm both a technical expert in HR and the house 'expert'/team leader in SAP Portal application developpement (Java), and someone that can look at both side of the connection (even if not an expert in both domain) is a very useful person on a developpment team.
if you go the java way, you have to know that, in addition to learning the language, you'll probably have to learn the "standard" java framework for SAP, webdynpro. While it's very powerfull, its also full of quirk.
Sap Developper Network possess a forum for both .NET and java where you can ask your questions.
regards
Guillaume