I am not familiar with that D50 add-on, but unless that provides phantom power, that is not something you want.
Do you record openly, or stealth? Or some of both? That will affect what outboard mics to add.
Assuming you want to run openly only, you have two basic options:
1. Keep D50, run mics>preamp>D50. Naiant (
www.naiant.com and a member of this board) sells a small box that provides 48V phantom power that runs about $300 depending on options.
2. Get a recorder that provides phantom power. There are now several decent options. The Marantz PMD661 is a bit more expensive but is high quality. Tascam also makes the DR-40 and DR-100mkII, both of which can do this as well.
Neither option is 'bad'. The D50 is a nice unit, probably higher quality than the Tascam offerings, more in line with the quality of the Marantz. You could unload it for about $400 and move straight to the Tascams without spending more, or spend another $250 to move to the Marantz.
http://www.bhphotovideo.com/bnh/controller/home?O=&sku=594741&Q=&is=REG&A=details My inclination would be to just add a preamp. The D50 is a high quality unit and you already know how to use it. The preamp is easy to get to know - there's not much to the tinybox other than an "on" switch and gain settings. If you get more into recording openly, you can always upgrade to a different preamp down the road, without ever getting rid of the D50. And the tinybox is, well, tiny - smaller than a pack of cigarettes. Carrying that plus the D50 isn't going to weight down your bag at all.
On mics, you really need to set a budget and be specific about whether you need to run openly or stealth, as you can get a fairly-good pair of mics for as little as $200 or so to an absolutely stellar pair of mics for $3500.
The D50's internals are, for all I've heard, among the best internals, especially if placed and used properly. I know at least one professional who has used one for "found sounds" and other one-off type recordings that he has actually incorporated into professional productions. But, you will see a large jump in quality if you acquire a decent set of outboard mics - especially since you won't just be stuck with the fixed X-Y or "wide stereo" patterns of the D50.