Of course the obvious choice is to try the deck's internal mics deciding how useful for interview. There are many external handheld mics you could choose for interview purpose considering the following for
sometimes adapting Sony D50's mic input jack.
Sony external mic input is mini stereo type with slide switched 'plug-in-power' on/off supplying up to 2.8 volts for electret type mics accepting this low powering voltage. So you might try a wide variety of mics with mini-plug output with/without their own battery powering with D50 deck.
Sony's mic input is
VERY quiet having an excellent mic input preamplifier inside. So even very low output (dynamic) type interview mics should work well enough with the D50.
Standard handheld
mono 'interview' type mics are usually
balanced XLR-male output types needing a cable patch adapter
unbalanced-wired to plug into the Sony deck's 3.5mm mic input.
I make a 5 foot long patch adapter cable (M-XLR-F) for this purpose and over a 18 year period of time this adapter cable has found use on many hundreds of minidisc, DAT, and now small flash decks with mini-stereo input jacks for public radio and other interview requirements using a
wide variety of mono microphones like the Beyer 58 and other makes/models of mics useful for interview purposes.
Most recordists choose a somewhat directional type microphone so the distance from an interviewed person can be increased while the mic more or less rejects outside interfering noise.
Even a shotgun mic of some kind is sometimes used, but these have more or less very narrow focus and are increasing harder to aim for best voice sound, so be sure to monitor with phones until you know how to distance and point any and all types of directional mics.
Remember to get a windscreen for whatever mic you decide is most useful and affordable.
See this at:
www.sonicstudios.com/access.htm#adapters