I've been doing a lot of searching online and browsing topics from all kinds of forums, all the while writing out an ever-growing list of lenses that people use so I could look them up and read about them. It's kind of overwhelming! What has been your favorite lens? And what do you use it for?
I'm kind of a minimalist, hence wanting a small more portable camera. I have a few lenses for my canon rebel xt, which are the tamron 90mm and the canon 55-250 telephoto zoom. I like both of them but they are the first lenses I've ever used for telephoto and macro work.
I have the OM-D, or EM-5, and its the first micro four thirds that I've bought. Of course, its fantastic. It has many features that make it excellent.
But cost is not one of them. Eventually, they will bring out an all plastic body, that will still have the computer power and the IBIS technology that makes the OM-D more special. Having weatherproofing is great, but most camera makers do not bother. In fact the OM-D is far more weatherproof than big pro Nikons are. But ... you don't need that either.
And you mentioned portraits. And having a small camera. The two do not really go together. Unless you have a big budget, and can buy a Leica (which are small but can have big sensors).
Really, for portraits, a full frame (35mm sized) sensor does provide more fuzzy backgrounds more easily than smaller censored cameras, such as micro four thirds and APS-C sensors found in most Nikon and Canon and Pentax etc DSLRs.
Of course, a full frame DSLR is huge compared to a micro four thirds.
So ... the way to get around the longer depth of field (less fuzzy backgrounds which assists subject isolation) is to buy fast aperture lenses. The faster the aperture lens (i.e. F/1.4 allows in twice the light of an F/2.0 lens and provides much more fuzz in the background than the F/2.0 lens does) then the more like a full frame camera will the micro four thirds camera behave.
Hence, I would recommend you buy a Panasonic Leica 25mm lens, which is F/1.4. And buy an Olympus F/1.8 45mm lens. And then, after you buy those lenses, then buy the micro four thirds camera that you can afford.
And if you just want one lens for starters, then buy the Panasonic Leica 25mm F/1.4 lens, because IMO its a very special lens.
Remember too, that the lenses do loose value, but they do not loose much of their value. But the new cameras plummet in price. So ... consider hunting down an older Olympus for instance, such as the E-PL2, typically they cost new now, only $250 or so, and often with a kit zoom lens.
As far as third party lenses go, yes, they will work, if they have a manual adjustment for aperture on their lens, and also a non electric i.e. manual focus on the lens.
An adapter for such lenses, can cost around $30.
Be aware though, that having a camera with an in built viewfinder may assist in manual focusing ... and that adds many issues!!!!!
As far a portraiture goes, a longer lens can also give you better fuzzy backgrounds. Some older lenses can achieve this, and they can be cheap. For instance, they are many many 50mm old lenses that have fast apertures i.e. F/1.4, and their size becomes on m43 equivalent in zooming, to a 100mm lens on a Full Frame camera (35mm), which is ideal for portraiture. However note: that on an m43 camera, the blurring of the background will be much less, than on the Full Frame camera.
And --- you can buy a FF Canon 5D second hand, for $750. Its pretty big, but for portraiture, IMO if the bulk can be handled, most Pro photographers would say its worthwhile.
http://www.bhphotovideo.com/c/product/800885633-USE/Canon_0296b002_EOS_5D_Digital_Camera.html