I've always felt that Roland does strings well - and they have so many synths on the market you could likely find any number of used keyboards fairly low cost. Since you don't talk about piano, I suspect you don't care about weighted action, etc. - which gives you a lot of low cost options.
You should be able to get by with a 61 key (or even 49 key) board - and candidly, in addition to Roland - Korg, Yamaha and even some Casio keyboards can do decent strings.
I haven't tried any of the recent Roland Juno series - but the older JP, XP, XV series all had several decent string options. Even going waaaay back to the D50 Roland had decent strings.
That all being said - if you do by a synth - give some real thought to what else you may think you could use (piano, organ, brass, etc). Once you have a synth you may start wanting to use other sounds (no sense buying a synth only for strings) - so....if your gonna spend the cash, try to find other sounds you can use (beyond strings).