I would say that even you go with the intention of not buying one, you should not rule out that this might happen as you might find one that you fall in love with. IMO, having gone shopping with no intention but then buying my dress 20 months out, I would advise waiting a few more months if you can. The wait to wear mine has been long and emotional and even though I absolutely love my dress, it is so hard to remember that in the middle of the long wait until your fittings start. Also, lots of new styles come out and you wont be able to stop seeing new and interesting styles on forums/tv/amgazines etc and you do question yourself about whether you prefer new things.
Why don't you get started on other things, there is still plenty you can do with 18 months to go. MUA, hair stylist (you don't need to know styles at this point, just get a good one booked), videographer if you are having, caterers if this isn't included in the venue, florist, cake maker. I had all these things booked at or just after the 18 months to go mark.
And if you want to get into dresses, just start familiarising yourself online with the designers and different options for each component of the dresses i.e. silhouette, neckline, waistline, details, materials.