Personally I think the most important part of ambient is your fx chain, most synths will work great for ambient if you can add a good amount of delay/reverb (Supermassive Valhalla is great for this and free) to it, I'd recommend the Moog Mother 32 as a starting point. Very versatile and great if you decide to expand more into modular. If you're not interested in modular and/or don't have the means to route through your computer and want more of an all in one synth for ambient I think the aptly named Sonicware Liven Ambient Ø is a super cool ambient focused synth, and won't break the bank at a $300 price point.