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to IIS Brent thats a good starter scope by the way so enjoy it , as it is for now and if you can afford it keep the 20mm and grab that good 25mm in the classifieds , if its still there but if not keep an eye there as they do show up often .
Then as said by Wavytone a 14-15mm ( the 15mm Plossl in the classifieds would be a good buy ) later on is good advice , as in my opinion unless you want to spend $100 plus ( TelVue ) on a quality barlow you might be dissapointed
as some of the cheaper barlows out there are not the best in shorter focal length Newtonion reflectors like yours , that is are designed for long focal length refractors where they work ok .
Yes a 10 , 15 , 20 and 25mm plossl set would be a great starter set that will last you for a long time ,,,,
until you need that 6-7mm and 32mm and ,,,,,, it will happen so be careful .
Good luck .
Brian.