Gday Marc. I'm relatively new too, and have lots of questions of my own, but I'll have a go at answering some of yours.
If you're planning to do an EJ conversion straight away, you may as well look for a cheap L with high km. Mine was $250 unregistered with a decent body but tired carby engine. You can then get a wrecked car at auction for as little as about $400 (early Lib with EJ22), or up to $1500 for a later Impreza or Forester with an EJ20. The other option is to keep an eye out for someone selling an engine, loom and ECU (and whatever other parts like exhaust and radiator) - that's how I got my EJ18.
You can buy an adaptor plate from the Backyard Boys
here, or make your own out of aluminium plate as Subafury has. You'll also need to modify your flywheel to suit the bolt pattern in the EJ crank (assuming you keep your original gearbox). Some people file out the bolt holes; I took mine to a mate's workshop and did it on a mill.
And as Andrew said, you'll need to bring your car's fuel system up to efi spec, which is what I've been working on all day. Personally, I wouldn't pay extra for a MPFI L just so I can rip that engine out - upgrading a carbied car isn't a big deal. The carbied cars need an external high pressure fuel pump added - I've got one from a VN V8 Commonwhore, as it was the cheapest external EFI pump i could find. Also, any rubber hoses between the new pump and the engine must be replaced with EFI-rated hose.
New front struts will be around $250, dunno about rears. It's probably a good idea to also budget for a new $300 heater core. Your $500 sundries budget should be ample provided the car you get doesn't require too much other work at the same time (like CVs, rust repairs ).
All up, I'm aiming to have my L on the road with a legal EJ18 for $2500.
Have a look at doing the wiring yourself. If you have to take the harness to a sparky, it could get very expensive. Going through it slowly and methodically it's actually pretty straightforward.
L series are part time 4wd - front wheel drive until you move a lever to engage the rear as well, and move the lever again to put in low range. One key difference between early subies and other 4wds is that they don't have a centre diff, so if you have it in 4wd on a sealed surface, things will break.