Welcome TW! Remember the OAL includes the handle obviously. 18" is fine, but for the work you describe I would go with a 20" anyway. Gives you a little more reach but is still very handy. My first was a 21" chitlangi, which to this day and many more knives later is still my favorite. I only mention the increased length because of the brush cutting task; for limbing trees I don't know that the extra length would really be that much of a factor, and anyway that's just my preference. Every body is different and 18" may fit you better.
One of the villager Gelbu's at about this length would also make a wicked and strong work knife. And of course the sirupate you suggest. That is a fine knife, and they have the nice brass inlaid sword of shiva, but to my hand they don't balance quite as well as the other two models I mentioned.
All three are usually between 27 and 29 oz. in the 20-21" length, and of course you can get them in the shorter lengths as well. Drop Yangdu an email if you don't see one come up soon on the DOTD.
Best of luck, and let us know what you decide on.
Regards,
Norm