Depending on the type of fabric and what you are making you could do either.
If I am making something woven or something that needs a seam, and then the seam allowances need to be pressed open ie. a skirt where I want the seams pressed open so they lay flat. I will sew 1st, serger after.
If I am making something with a knit fabric... it depends on the knit, if it's thicker, I'll overlock it and leave it, if I think it's not going to stand up to just overlocking then I'll sew and overlock.
You will have a seam guide on the overlocker, similar to a sewing machine, so you would just follow the guide and sew a 3/8" seam like you would on a sewing machine.
I have a pink Kenmore 3/4 machine and use it for taking to classes. It is plastic, but it has a free arm which I like for hemming.
I know the Janome Jem is a good machine as well.
There is a Singer 3/4 machine which is a Featherweight which was manufactured in the lat 40's early 50's. They are highly collectable, BUT if you look you can get one at a price you are willing to spend. They are well sot after in the quilting world, because they are simple, and the foot they use is a perfect quarter inch.
The model after the Featherwieght is the 301, and it is still compact, has a few more stitches, and is also A LOT cheaper! The only down fall, is that it is a slant foot which they no longer make, and finding accesories for it is hard.
I have a Singer Genie from the 70's, which is not a 3/4 machine, but is 19lbs, and comes in a case which covers the machine and the foot and cord fit in the top. It is very compact. I found mine at a local church for $15, but they do sell on ebay quite cheap.
I have a book called "Weekend Sewing" and the author Heather Ross lives in NYC as well, and she has a featherweight, and also a Necchi Syliva Maximatic. I am not sure how hard they are to find, they look compact, as well as having some stitchs other than just straight.
I had a look at all of the machines, and the ebay Brother had ended.
The ebay Janome, has to be mounted in a cabinent.
The craigslist Brother looked good from what I could see, certainly it is a good price, have a look on google to see what people are saying about it, that would be helpful too. She is including the accessories, so that might be worth it right there.
The Brother from overstock seems to be a good deal too, I have the basic Brother from Wal-Mart, and have been happy with it. I belong to a quilting board, and people are fairly happy with those Brothers too. I believe this machine is at Wal-Mart too, but I wouldn't quote me on that, but it's worth checking into.