James,
WLC's are fun (I've had three) and they aren't especially rare but good ones are hard to find. Bikes lead a rough life and the surplus ones were cheap transportation and they were cheap during the "chopper craze" so make sure you check the frame of any prospective bike. Bruce Palmer's book is the best resource to find out how they left the factory. Please pay attention to the bikes set up for "Domestic" and "Overseas" configuration. I always liked the overseas configuration where the passenger seat was eventually replaced with the radio rack and saddle bags, windshields and leg shields became pretty common. Also remember that WLC's had left hand throttle and right hand spark advance, opposite to today's bikes. Another good book is the "Winged Wheel Patch" by Max Burns.
There are lots of NOS and reproduction parts around to help you restore one but it will be a challenge on "Troopers" pay while in the corps (I'm assuming you're not joining as an officer, but even officers get paid peanuts in the beginning). Even rough bikes in need of a full resto are in the $7K and up range, a rideable bike may be $12K or more. The "Harley effect" in full bloom.
Once the bikes were received into the units any number of changes were made to make them more useful. As the Harley's are a heavy bike and not really suitable for serious cross country use they were preferred for on road dispatch riding rather than cross country with fwd units. Most front line units got Norton 16H's when they could and that's why you see so many WLC's in wartime photos with markings from higher headquarters and MP units.
Some parts sources:
https://www.nosparts.nl/
http://www.45restoration.com/default.aspx
http://www.jpcycles.com/1942-harley-davidson-wla
https://www.flatlandmotorcyclecompany.com/
This book you must own!
http://modernmotorcyclemechanics.com/
Written by J.B Nicholson who developed the full CDN Army motorcycle mechanics and rider program. This book was also the textbook used in Canada, the US and many other countries as the bible. A worthwhile investment and lots of 45 info.
Good luck, the only time you'll regret a WLC is making a left turn uphill into oncoming traffic! That foot clutch can be a pain in the ass.