I haven't worked with down yet, but I have heard it isn't too bad. If you want to make it under 18oz, that is the easiest way. This would take say about 8 to 10 oz of down, depending on the size of the quilt and how confident you are to not lose any of the down in transferring. For my quilt size, probably 8.8oz would be the final weight of down. So this would cost about $60.
I personally really enjoy synthetic quilts. The 5oz Apex gets me down to 30-35 with proper layering. My quilt with just 30d ripstop nylon comes in around 19.5oz. The total for the quilt, including shipping, was $70, but this didn't quite hit your weight limit. You could fix that by upgrading to M50 or possibly a 5mm silk liner. The synthetic quilt is very easy to make in my opinion. I wrote a guide about it here
Either project should be a good project, even for a beginner. You can accomplish it for less than $90 I would say regardless of which route you go.
One suggestion I would make is that for 30-35*, you should avoid sewn through baffles like in Jamie's design. I normally hear that there are too many cold spots in a sewn through design.