I guess this is where you really have to determine what you want. You want to shoot 1000 yards. The one decent thing about 1000 yards, is that a great rifle and a bad shooter looks just about like a good rifle and a bad shooter. If you are a bad shooter and want to learn, there isn't a huge need to move to a great rifle right off the bat. It won't hurt, but it may not help much either.
I would buy myself a decent .22lr and spend a ton of time at the 200 yard line. Practice that for a year and shoot a few thousand rounds. You'll gain a lot of knowledge and only spend a few hundred on midgrade ammo. I would spend $500 on a used target rifle from a few years back, $400 on a case of Wolf ammo, shoot for a year, sell the rifle for the same $500, and be out $400 for the experience of 5000 rounds at a relatively long range for the round's ability. Take what you have learned and build yourself the rifle you wanted and be a bit more able to actually use the rifle to it's potential.
Look at what shooting 5000 rounds of hand loaded .223 would run you. At that, you'll go shoot a barrel out, if not 2(for a match rifle, it won't be useless at that point, but won't be giving you scores like it started life). A good rebarrel runs an easy $500. A couple hundred cases, lets say 500 cases shot 10 times each. For cheap brass maybe $100, for Lapua $200. 5000 primers at $25/k would run you $125. For powder, lets just say you use Varget and run 23.0 grains (not real sure the load data for the ultra heavy .223 bullets). 7000 grains in a pound, 16.4 lbs of powder/5k rounds. $140/8lb jug x2 is $280. Say you go with the 80gr SMK. At the 500 round quantity you are looking right about $100. So for 5000 you would be pushing $1,000 worth of bullets. Let's assume you get your entire cost back out of the rifle itself, but with a rebarrel at the end of the year.
So $500 for the rebarrel, $100 for cheap brass, $125 for primers, $280 for powder, $1000 for bullets. That's $2000 and I tried to round those prices down for an optimum use. If you want to move up to say .308 prices of components seem to rise a good 30% from there. And this ignores the cost of reloading equipment and the time invested in making the ammo, which at 5000 rounds, is intensive.
If you go out and buy FGMM it seems to run about $1 a round. Even if you could find it for $0.60 a round, that would put you at $3000 worth of ammo and a $500 rebarrel.
I am all for you buying the rifle you want to buy. But, if you really have no experience with shooting rifles at long range, try a year/summer/whatever out with a .22lr. If you find you hate the game, you won't be out all the time and effort to have a custom rifle built, ammo bought, etc. If you find you love the game, then you have gained 5000 rounds of experience in doping winds, which will be a huge advantage. It isn't the same thing, but it is similar and at a minor fraction of the cost. The experience you take from case of .22lr will be worth more than a couple hundred rounds at 1000 yards.
But if it must be 1000 yards right now, I would got .260 or .243. Either will get there. They won't be the absolute best rounds to do so, but they are fairly easy to come by and with the right barrels/twist/loads should do just fine at those ranges.
