Re: Advice on buying a new mandolin? Been playing for 15+yrs
IMO, it's very difficult to get a "lifer" mandolin (especially with an installed pickup) for $600. The pickup itself usually is at least $100 with the installation (sometimes more).
If I was in your shoes, I'd buy another "right now" mandolin that was the best I could afford. A few from the classifieds
https://www.mandolincafe.com/ads/175659#175659
https://www.mandolincafe.com/ads/175634#175634
https://www.mandolincafe.com/ads/175608#175608
https://www.mandolincafe.com/ads/175605#175605
They won't last forever, but they will do the job. Ideally you should practice your setup techniques on this too. Most often the problem with lower cost instruments isn't bad production, but bad / inconsistent setups and more frequent maintenance needs. If you can do it yourself, it'll be cheaper for one - but you'll be a better mandolinist as you'll know your instrument better etc.
Immediately after I would start saving for the lifer (or at least "long term") mandolin. When I was saving for my Gibson F9 I put away about $200 / mo in a "mandolin fund" and tried not to touch it (didn't always succeed). Took about 1.5 years but I eventually saved up enough to buy the F9 I wanted with a pickup installed and the fingerboard radius done (thanks Mandolin Store!).
Re: Advice on buying a new mandolin? Been playing for 15+yrs
Sue briefly mentioned, but there are a couple of Morris mandolins in the classifieds. Yes, even the flat tops are a touch above your price range, but I do consider them to be very good instruments.
Otherwise both the Eastman and Kentucky models linked above are good choices. Have liked the Kentucky KM-150 models I've tried. Again, if you can stretch your budget, I'd say get a Kentucky KM-500. Have heard and played a couple and they are really good.
Good luck in your search.
Re: Advice on buying a new mandolin? Been playing for 15+yrs
I bought my Morris brand new. The only brand new instrument I have. Prices have increased a tad since then. That said, there is is this in the Portland, OR craigslist for $585.
Re: Advice on buying a new mandolin? Been playing for 15+yrs
Look for a used or near-new, Morris oval flat top. These have great wood selection, have wonderful tone, and look beautiful. Made in Oregon by a Howard Sonny
Morris. A great guy and a skilled luthier.