Strat recommendations are heavily discussed
here.
As far as a production Fender Stratocaster my suggestion is still the Eric Johnson model.
Money no object....a Masterbuilt Relic.... or a "real" vintage instrument (which would probably be played sparingly considering it's probably worth more than a sports car).
What to look for? Consistently I would say that classic strats have Nitrocellulose finishes.... which are more expensive and tend to show age more than the polyurethane or polyester variety.
There are quite a few different neck radius'....I've heard them described as C, D, base ball bat, standard thin...and a couple of nut widths (1 11/16" or 1 5/8" inch). All of these are clearly a matter of preference. I tend to gravitate towards a C neck (standard thin) with the 1 11/16 inch nut width (like my 2002 American Deluxe)... but I'm not dead set. It's about the "whole" strat for me.
Maple vs. Rosewood fingerboards (or Ebony...or Brazilian Rosewood).... this is also a matter of preference. Maple tends to highlight the players' wear pattern as it ages....and there is a certain coolness to it in the relic/collector market when it has a cool wear pattern. Although the darker woods tend to have a certain look (because the fingerboard is usually not finished over). If blindfolded, could you tell the difference between maple and rosewood fingerboards? I have friends that claim they can....I know I can't. I think its a "look" thing. Also, the finish on the back of the neck makes some difference on playability to me (satin...thick glossy, no finish). Again this is a matter of preference. I tend to gravitate towards very little or no finish (although it makes the neck look pretty beat up over time). I've played amazing strats with both maple and rosewood fingerboards.
Hardware....there are different camps here. If you want a vintage instrument...then the hardware will be aged, and perhaps a bit more difficult to care for (less precise machine heads....more susceptible to rust over time)... but it gives the instrument a more vintage vibe. Modern hardware makes the instrument more easier to setup but will probably not wear as much over time (of course, only time will tell). I have friends with 80's era strats that still look good as new.
Then of course there are the pickups and the famous 60 cycle hum. I have a love hate relationship with vintage pickups. In the right situation, they can sound like nothing else.... but in the wrong room...with lots of interference they can drive me bananas...and have me reaching for a different instrument altogether. Fender over the years have tried various noiseless varieties (SCN-1, Vintage Noiseless, Lace Sensor).... but there are still devotees who insist that there is nothing like a real strat pickup (or even hotter versions like the Texas Specials). This is also a matter of preference. I tend to favor the Texas Specials or the Vintage Noiseless.
Then probably the most important determining factor in determining the strats worth is where and when it was made. Is it pre-CBS, made in Japan, Mexico, Korea, Corona or simply the USA. There is a saying that a guitar still thinks it's a tree until it's 25 years old....where it realizes "Hey, I'm a guitar" and gains that vintage vibe..... I tend to agree with that statement....although, with too much finish makes them seems less vintage.