I agree with most of the advice given.
1 - I have the Vision Hudson. It is noticeably pricier than Legacy locos but if you can stretch, worth considering: best sound, period, and nice features and great runner.
2 - the S3 is nice and a real bargain at some of the prices I've seen.
3 - the new SF Northerns (3751, 3759) have the whistle steam, etc., and look impressive and sound great.
4 - the 2-6-6-2 Mallet is listed as running on O-54, I have one and it runs nicely on 60" and I doubt Lionel would lie about that. It has good sound and all the recent features.
You mentioned O-54 curves. Sometimes a loco will run on a diameter but not look good on it, and frankly I think that might be the case with the the northerns, particularly the SF one, and the Mallet - its boiler front will stick out well over an inch beyond the outer rail on curves. I have all these locos and they look okay on my 60" radii curves but even the northerns a bit "big" and ungainly around curves - I keep them ont he72" loop I have. The Vision Hudson, is the only loco I list above that seems 'at home" on 60" curves (I have no 54" but I imagine its about the same).
Something to consider is the new Pacifics. They have all the new features and really good sound, etc. I know you have a Southern Crescent but it comes as the Blue Comet or another more traditional painted roadname, and it is of a size that looks good. Mine will run on 36" curves but looks a bit ungainly around those curves, but it does nicely on 48 and up. The Legacy 0-8-0 is worth considering - and rather the least expensive among Legacy locos, but will not have the whistle steam effect and some of the advanced features, and a bit smaller than you probably want, but a nice loco nontheless.