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Definitely try those eyepieces out from John Briggs. The Erfle will be pretty "awful". The 13mm Nagler will be very good and at least you will get your head around what a quality eyepiece does.
Some newcomers have trouble getting their head around the concept that in many cases a high quality eyepiece costs as much as their telescope did. You can find some bargains in the telescope world where the telescopes performance doesn't correlate to it's price. Your 8"/F6 dob is one of those. It is a reasonably cheap telescope but they perform very well and well enough to benefit from quality eyepieces. Unfortunately the same thing rarely happens with eyepieces. Cheap eyepieces are generally pretty poor and you usually get what you pay for. Something to keep in mind is that cheap low quality eyepieces have a very poor resale value, whereas high quality eyepieces will always have a reasonable resale value. A new $150 eyepiece, would be lucky to be worth $30 in 2 years time, because it's an eyepiece no one wants. A high quality eyepiece on the other hand will be worth about 75% of it's new price. If you can buy high quality eyepieces 2nd hand, you can almost get your money back on them. To give you some examples, I bought all of my Pentax XW's new for between $AUD 400 and $AUD 440 with some smart shopping, about 20 years ago. There are 2 currently for sale in the classifieds (a 7mm and a 10mm) for $AUD 380 each. I bought my 27mm Televue Panoptic 2nd hand for $300 about 16 years ago. There is currently one listed in the classifieds for $480.
Hard to know what eyepiece you should buy because you haven't really worked out what you like looking at. Over the next couple of years you should look to aquire some quality eyepieces that you can keep for a long time to come that will also work well if and when you upgrade your scope. In time and as an absolute minimum you will need a 2" low power finder eyepiece about 30mm focal length. Something in the 10mm to 12mm range and something in the 6mm to 8mm range as a starting point. The next one in the list would be something between 16mm and 20mm and something high power around 4mm to 5mm. You could also get a decent barlow which will double up on your 1.25" eyepieces.
Over several decades I've found that my 2 most used eyepieces are ones having a ~2mm exit pupil and one having a ~3mm exit pupil. With my F4.5 to F5 scopes that is ~10mm and ~15mm. From my couple of dozen premium eyepieces my 3 most used eyepieces are the 10mm Pentax XW, 14mm Pentax Xw and 17mm Nagler T4 When combined with a Paracorr which has a 15% amplification factor that ends up at about where I want to be. I find those to easily be the best options for general observing of the brighter DSO's including Galaxies, Globular Clusters, Open Clusters, Emission Nebula and Planetary Nebula.
Some of the best value for money eyepieces at the moment are the APM UFF if you are buying new. If you are looking 2nd hand there are some excellent options including Pentax XL and XW, most things from Televue with a couple of exceptions (eg 22mm Panoptic which is clearly inferior to the 24mm Panoptic), Vixen LVW, Baader Morpheus etc. As I said earlier, don't be scared to buy 2nd hand to start with. That basically allows you to try out the eyepiece for a small rental fee.
Cheers
John B
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