You are right. The first thing you should always buy with a serious telescope
are more eyepieces. Serious telescopes
are never supplied with more than one eyepiece - the manufacturere assume you will want to choose the eyepieces that best suit your observing needs. However, you definitely need more than one eyepiece to cover all the bases in astronomy. Does not have to be the same brand as your telescope.
I generally recommend a range of magnifications in the low, medium and high range. 300x is only going to work for you if you have a great site on a nite of really excllent observing. You'll probably get more mileage form an eyepiece that comes in between 200-250x for the high end. Usually, three eyepieces or two eyepieces and a barlow will do as a minimum Use high magnifications for the moon and planets, but keep magnifications on the low side for nebuale, star clusters and galaxies.
A good scope like that deserves to go with some good eyepieces. I would start with Meade
's better eyepieces, the Series 5000 or any Televue
and so on. Basic Plossls in each brand will get you started, nicely. go with the wide angle designs if you want them and if you can afford them.
eyepieces are the most important accessory for any telescope, you may also wish to do more reading with my article, Ten Telescope Eyepiece Questions