Panasonic 100-300 a good lens?

If the lens is excellent between 100-200 IMO I think the 45-200 is a good lens and you can spend $300 less and get the 45-200, its also smaller.

Then you have to decide how important that 300 mm is.

If your committed to spend $500 then you have $300 to get another lens, better tripod etc, software.. re-direct the funds and get double the benefit on something you were really waiting to buy.

The 100-300 is a lens you have to think about using its not an everyday lens, its a speciality lens IMO.