I was under the impression that you already bought the 30. If not, that 500 might work better for you, although I think 300 bucks is a tad on the high side...considering you said it has "cheap electronics"
Look at it this way....
HK sells the 500GT in a kit form (plastic frame edition, belt drive) with metal head upgrade and main blades for 65 bucks plus shipping, see link...
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=9975
Or you can get the CF edition which comes equiped pretty much the same but add torque tube tail drive and metal tail with of course CF frames for 99 bucks. See link...
http://www.hobbyking.com/hobbyking/store/__14862__HK_500CMT_TT_3D_Torque_Tube_Helicopter_Ki t_Align_T_Rex_Compat_.html
Ehirobo sells an el'cheapo electronic kit that comes with Gyro, 3 cyclic and 1 rudder server, motor, and ESC for 100 bucks....linky...
http://www.ehirobo.com/shop/product_info.php?products_id=8477
Only thing lacking is a battery...which you can get a decent cheap 5s 4000mah pack for about 50 bucks just about anywhere.
Add all of that up, no matter which platform you start with and you will be in a brand new one for close to the same price as this used one for 300 bucks.
If I were you...I would offer 200 bucks for it and see what is said. As I think that is a "fair" price to offer for a used clone 500.
The 500 is about half the size of the 30...but parts will be available almost anywhere as you can use Align parts on the HK500.
I dont have an HK500...but do own an EXI500 and I absolutely love that heli. Its one of my favorites and its much nicer to fly than my 450 and smaller helis. I run mine on 5s 4000 packs and have 4 of them. I can easily get 10 minute or longer flights on each pack of normal flying.
Some, if not most people like to run their 500's on 6 cells. But for me....5 cells is plenty of power for my skills right now. Plus, I am still running a cheap motor on my 500 and Im not really sure it could take the power of 6s.
No matter what you decide, goodluck with your pursuit of this addiction we all call RC helicopters! lol