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Next question is how much do you want to spend on eyepieces?
12"/f5 = 304.8mm diameter, 1524mm focal length.
May I suggest a 9mm Type 6 Nagler. This will give you 169x and 1.8mm exit pupil and 0.47 deg true field at 82 deg apparent field. This will be a great eyepiece for planets, globular clusters, planetary nebulae and galaxies.
Have you tried to Barlow the 18mm ES to see what 169x will be like?
To complement the 9mm Nagler may I suggest the 13mm Type 6 Nagler. This will give you 117x and 2.6mm exit pupil and 0.66 deg true field at 82 deg apparent field. 0.66 degrees also allows for a full lunar disk which is nice. Anything under about 0.65 deg and the Moon only just fits in and a little bit of space around it is nicer.
The 9/13 Nagler combination is nice and the Type 6 Naglers are quite compact and wont cause balance issues.
An alternative would be either the 11mm Type 6 Nagler, 10mm Delos or 10mm Ethos but you can buy two Naglers for the cost of one Ethos. Up to you what apparent field you want. Note there may be some nights you want more power than the 13 can give but the 9 is too much in which case the 11mm Type 6 Nagler is handy.
Note if you need long eye relief then go the 10 or 12 Delos or 12mm Type 4 Nagler.
If you use a Paracorr increase magnification by 1.15x and decrease true field and exit pupil by 1.15x.
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