Honda Shadow VLX (583cc). - a bit larger than you want, but not by much.
Long production run
Available in a variety of engine sizes
Manageable power delivery
Shaft drive for low maintenance
Comfortable for passenger and rider (especially with an added "sissy bar" and an aftermarket seat - I recommend Mustang)
It's a Honda and will run forever.
Don't get so caught up with engine size, it's power that you should be looking at. Some smaller engines are tuned to deliver more power than other, larger engines. Twins will be different than inline 4s which are different than V4s. Cruiser motors, like the one in the Shadow, have a very different power delivery than the 600cc motors in sport bikes like the CBR.
What you probably want as a first bike is a linear power curve. Some engines produce nothing until they hit 4,000 or 5,000 RPM and then it explodes with force. I like my 750 V4 because it has such manageable power delivery. If you are both wanting to ride together, I doubt a 500cc will be good enough for you. I have ridden pillion (2-up) on a 500 but the bike was at the limit of what the motor could do. You really need to have a power reserve to run away from danger.
See if you can find a late-model Magna. They were made until 2003, while they are a 750cc motor, the power delivery is very forgiving, especially in the low end. Also, you won't get bored with it quickly. Its a
very capable motorcycle and has a devoted following.
Also, if you have not learned to ride yet, take the MSF course and start out by buying a cheap old bike that already has cosmetic damage. Use it to get the feel of riding for a few months and then sell it to another beginner.
What kind of riding do you want to do? Long distance cruising? Sport-touring rides (2-lane blacktop, plenty of sweeping turns)? Around down "putting?"
Also, price out what gear you want to buy. The gear is something everyone forgets, but it is vital! A good helmet, a heavy jacket with shoulder and elbow armor, a back protector, armored gloves that cover the wrist, knee armor and abrasion-resistant pants (leather or textile) and riding boots. Check out
http://www.newenough.com