Olympus are concentrating on m4/3. While there are still 4/3 lenses available, they are mostly on online auction sites.
Their lens lineup has 2 macro lenses. The 35mm f/3.5 and the 50mm f/2.0. The 50mm is the better, more expensive one.
As for the Sigma 18-50mm, I haven't heard of it before. Are you able to find a link about it so I can properly compare and answer?
The link in your answer doesn't state it is a Macro lens. Most macro lenses are a fixed focal length, also.
In regards to getting rid of the 4/3 glass, there is no need. Olympus have an adaptor for m4/3 to 4/3. The MMF2 and the MMF3. The MMF3 is meant to be waterproof but there is no speed difference with these.
What makes the difference is the body. That adaptor will work on any m4/3 body. Of course designed for Olympus bodies, they will still work with Panasonic.
The OM-D, E-M1 was released as an alternate to the DSLR and the focussing speeds with 4/3 glass is almost on par with the E-3/5. Certainly with the E-620 and faster than the 4xx/5xx series.
That sigma is actually designed for the APS-C format (think specifically for Nikon's DX format, not Canon's APS-C format), so yes, it will work.
I guess their isn't really an answer for "Keep lens, buy body" or "Buy new lens only" as that will come down to personal preference. Research, take a look around. Try out the E-M1 body and Nikon's DX bodies and see if you can get your hands on the 50mm f/2.0.
But, both options are available.