If you're looking for high-quality pictures and ease-of-use, this isn't the answer. If you have some old lenses that you want to use and have fun with, then you should definitely get a mirrorless camera and an adapter.
With this setup:
You will lose EXIF information:
- focal length (a big deal if, like me, you change lenses frequently: you can no longer identify which lens you used by the EXIF data)
You will lose features:
Depending on the camera, it will be harder to change settings. It takes several button presses to change the IS mode, ISO, or Ev (or the shutter speed when in manual) on my E-PL1. There's no dial for any of those: I have to hunt through a menu to get to them. It's frustrating when I just want to take a picture right now, but I love using these lenses so I put up with it for their sakes.
All of the things you lose are, really, just conveniences. But conveniences are what make more expensive cameras worth paying for.
The Nex's crop factor of 1.5 isn't terrible. If you have that 40/1.8, it'll be like a long-normal 60 instead of a wide-normal 40, so you'll live. I am using it with a 2x crop factor E-PL1 and it's definitely not a normal lens. For one important shot, I took several shots in portrait orientation and stitched them together later -- I do not want to make a habit of that, though. I don't have anything wider, so I can't tell you what it will be like to carry around a 24mm or 28mm lens.
My E-PL1 has in-body image stabilization which is fantastic for manual focus lenses because it actually adds a feature. It also has a dedicated button for zooming in to the center of the image (I have it set at 7x, I think it goes up to 11x?), and although I have to hit the button twice to zoom and then hit another button to exit the zoom mode, it really helps me get the focus right.
With no optical viewfinder, I've had no luck at all trying to capture fast-moving subjects like birds.
The biggest downside to your plan is that the lenses you have just aren't that good. Modern lenses have much better coatings and will give you much, much sharper images with better contrast and less glare. Of course modern lenses are also lighter, often have image stabilization, and if you pick the right ones they also zoom in and out. An f/4-5.6 lens wouldn't beat my 40/1.8 on speed, except that I get a lot of glare with the 40/1.8 wide open and have to stop it down to f/4 to get results that I'm happy with.
This action shot is from my first real outing with my E-PL1, and cropped and then resized to 50% (further resized by StackExchange for inline display; larger version), but otherwise untouched. I assume it's with the 80-200 off-brand f/4(?)-5.6, probably stopped down one stop because there's no flare. With no supporting EXIF data, of course, I can't verify any of that. 1/200s with in-body stabilization (again, fantastic), though I really wish I'd pushed the ISO above 100 to have a chance to freeze the action (live and learn). Well, I'm happy with the result -- and of course it was really fun to have a small, lightweight camera with a fully-manual lens!
(NB: that catcher was drafted by Pittsburgh today, in the 32nd round. Any Pirates fans here?)
So if you want quality or convenience, skip the manual focus lenses and get a cheap all-plastic consumer zoom lens with whatever DSLR or mirrorless camera you want, and you'll be really happy with the results. If you think the manual focus lenses will be fun and you don't mind spending more time to get the shots, then by all means get a camera and adapter for them. You'll have a lot of fun and if you're careful you'll get some great pictures!