What an odd coincidence! I happen to have a D-50 myself. In fact, I'm planning getting another soon. I will try to answer these questions for you one at a time.
-I don't know how to change the battery, but I can say that one in proper working order will maintain all the preset memory when it is switched off.
-I would try simply removing the bottom cover and seeing whether there is or isn't a battery in plain sight then. It will probably be a button cell type, (round shaped like a quarter). While computer knowledge is good to have, I doubt it would be of much additional use. While this instrument is digital, it isn't a computer. Knowledge of integrated circuitry would probably be best but, then again you're only changing battery.
-Indeed, the D-50 came with 88 amazing preset patches. A list of them with sound clips can be found here. Now normally from a synthesist's point of view, good presets aren't necessarily a good thing. They are often used as excuse to make the synth less tweakable, (ie: no knobs). In the case of the D-50, while that is still true, the preset patches are very good and can yield very good results.
-The memory cards are no longer available new, but they are out there. you can find them on synthesizer forums, Ebay, etc. The former would probably be your best bet. The memory cards come in two varieties, those with more patches and those without. Here is some info and the sound samples of the four patch expansion cards: http://www.synthmania.com/pn-d50-series.htm
-It is quite possible that the machine has lost the preset memory. Unfortunately, There is NO factory restore function on the D-50. 
I really don't know what they were thinking, as just about every other Roland project has it. 
Mine has been modified, and I am trying to find someone with one that has the factory presets that would be willing to do a data transfer so I can get my sounds back. I you find out that yours didn't save your presets, I might be able to send you my memory card so that you can load them on, worst case scenario.
-Yes you can use it as a MIDI insertion device IF, the hardware/software you connect it to will accept that level of MIDI implementation. Keep in mind that the MIDI on the D-50 was actually designed to control other hardware instruments. this was a long time before soft synths and Hauptwerk became available, IE 1987. Of course, I should not have to tell you that there are no drivers available for it. Although, it's possible that you could use a generic MIDI driver and have some success.
If all you want to do with it is use it as a MIDI controller, for the love of all that is holy in this world allow me to trade with you a keyboard that can do MIDI better and isn't such a classic treasure of synthesis!
Moving on:
- Prices vary. I got mine for $100, but Vintage Synth Explorer shows them in the 4-600 dollar range. They say the same about the Korg M1 though, and I have seen those go for under $250. Real world value? Probably in the $150-200 range in good condition, less than $80 if it doesn't work right.
Here are a few good D-50 links for you:
http://www.vintagesynth.com/
http://www.synthmania.com/d-50.htm
http://www.synthmania.com/pn-d50-series.htm
http://www.youtube.com/profile_videos?user=RadekTymecki&p=r
http://www.youtube.com/profile_videos?user=alphacode909&p=r
Good luck!
-Joe
Have: Allen 301, Crumar T1, Hammond E-311 + RT-2 + T-212, Korg DW-6000, Mason And Hamlin Style 802, Moog Micromoog, Rhodes Mk1 Suitcase 88, WurliTzer console, Yamaha E-70 + DX21 + DX7
Looking for: Yamaha EX-1/EX-2 or CSY-1/CSY-2