So for $600, the best bang for your buck is going to be a used.
Although, the BEST (I may be partial because it's been the only saddle that fits my pony) would be the Stubben Roxane... but those go for about twice what you say you can afford unfortunately :(
But here we go. I've seen some courbette's, m. Toulouse's, and pessoa's in the $600-$700 range.
Honestly, what you're really going to want is:
Wool Flocking- so that the saddle can be fitted to your horse
Forward Flap- better suited for jumping and the short-stirrup gallop position of XC
Deep Seat- better suited for XC but not too deep to get out of while going over jumps
Thigh Blocks- not necessary but definitely a plus as you progress upwards in eventing
Wool flocking is not going to be as necessary when buying used, as presumably the foam is already crushed down and it's either going to fit your horse or not, but having the wool flocking makes it more of an option if it doesn't -quite- fit but has the potential to and fits you well. Other than that, you just want good quality leather that's possibly worn in but not worn out. I just gave you some of the nicer options of saddles that tend to fall into your price range when "used" but I've seen some of these "used" saddles looking almost brand new so you might get lucky.
I would go with a used leather saddle. I always like to use a cc saddle, in the hopes of saving up for a dressage saddle when I move up the level lol. I really like m toulouse, and as they are not super expensive new, you can get great deals on used ones.
I event in an old school Stubben VSD... it's been a great all around saddle and is secure enough for XC. There is something about the old german saddles, they just don't die. I love the saddle, it's no frills, but you can find a gently used one for 400 or so.
But yea... if you can get an M.T. Marielle.. go for it.