Have a look at
https://www.bbastrodesigns.com/diagonal.htm and plug in your numbers. Use inches or mm but not both at the same time.
I used inches so I get inch sized diagonals as the metric ones are not available in the same sizes. I put in 18" for aperture, 90.9" for f.l. and 12.2" for distance from sec. to focal plane.
Now the field stop diameter depends on what you want to observe. Start with your lowest power eyepiece you intend on using to see what you get and adjust accordingly.
E.g. At f5 you may not want more than 35mm Panoptic which has a FSD of 38.7mm and will give you 7mm exit pupil at f5. Convert 38.7mm to inches = 1.5".
See
http://www.televue.com/engine/TV3b_page.asp?id=214 for FSD.
You will see with the above numbers you get a fully illuminated field diameter of 0.2" with a 2.6" and 0.8" with a 3.1" and 1.2" with a 3.5".
Or in other words;
2.6" has zero magnitude drop over 0.2" diameter circle and drops 0.4 mag at the edge of a 1.5" field diameter.
3.1" has zero magnitude drop over 0.8" diameter circle and drops 0.2 mag at the edge of a 1.5" field diameter.
3.5" has zero magnitude drop over 1.2" diameter circle and drops 0.1 mag at the edge of a 1.5" field diameter.
A 4" sec. would not drop any mag over a 1.5" field. Contrast would suffer but still at 22% by diameter a lot less than a typical SCT.
If your low powe eyepiece is a 31mm Nagler with 42mm then a 3.5" still gives less than 20% contrast factor and 3.1" is less than 18%. (The impact on contrast is compared using sec. dia/pri. dia ratio not by area). The lower the % the better and anything under 20% is excellent and quite acceptable up to 25%.
Now if you are a variable star observer you don't want any magnitude drop across the field of the eyepiece you intend on using so a larger diagonal is required. If you are a planetary observer you can get away with the diagonal that gives a very small fully illuminated field. In your case a 3.1" secondary is just about perfect for an 18" f5 mirror for general observing. If you had an f4.5 then I would consider a 3.5" secondary.
As for quality buy the best you can afford and remember the secondary has to be twice as accurate as your primary as it introduces an extra reflection. E.g. If your primary is 1/4 wave P-V, your secondary must be 1/8 wave P-V. That is why secondaries are often 1/10 minimum with 1/20 typical for the premium ones even up to 1/30 although the difference may be seen in only the best seeing conditions with the 1/30 and provided you primary is exquisite and optical train properly collimated. (Typically 1/20 is more than adequate).
You have to ask yourself how good is the primary and if of premium quality then you really should get a premium secondary. Your optical system is only as good as the weakest link.
See
http://www.antaresoptics.com/Products.html and
https://www.astrosystems.biz/secm1.htm
Note you also have to choose a secondary to fit your holder. The 3.1" and 3.5" are imperial sizes and typically fit a secondary holder that has a lip around the secondary. The GSO are metric and are not direct conversions in mm. These will not fit the lip type holders meant for imperial sized secondaries and are typically glued in place.
Hope this help and whatever you choose, enjoy!
