Hi Anastasia,
having been through the same process as you I thought it might be useful for me to add my experiences.
I am an acoustic guitarist/vocalist and for the same reasons as you I bought a Boss 900CD digitar recorder just over a year ago. It is sort of similar to your micro Br (though not quite).
As far as I am concern to Boss machines are great (especially the effects)and Im sure the micro Br will suit your needs. I would check out the operation of the unit before you buy it by looking through the instuction manual (which is on line at roland boss). Having said that the weakness of the CD 900 is the manual, it has taken me a year to be able to decifier it (it really is 5 year old dirty PANTS).
If you live in the southeast i would suggest you travel to brighton to the GAK shack (phone them up first and ask for the guy (think he is called brian) who used to work for Boss recorders. He knows everything but everything led me through everything step by step in a none technical way. You can also ask the people at Boss who i hear are pretty good (check their website for the contact number), there is also a Roland boss forum and there are always loads of questions fromm people on the micro br which are similar to your questions.
I believe that the there is a new dedicted Mic for the micro br. I bought a sure Sm 57 which I use for recording and for performing and can confirm the combination works well.
As for playing and singing at the same time, I think you can do this on the micro Br (not certain though) but you will need to buy a mic (buy the best you can as recording quality probibly depends more on the mic than anything else (er apart from your playing of course). This was origonally my intention but I quickly (well quickly for me that is) learned that you get much better recordings by playing the tracks seperately.... although it was a bit of a learning process but had the added advatage of improving my playing no end. Dont be fooled into thinking just becuase yopur only using guitar and vocal it will be easier, in some ways its actually harder than multi instuments.
I Hope this helps somewhat and gives you a shortcut to the many (enjoyable) hours it has taken me to get to where I am now....... which is not that far !
mikeshead