Another option would be simply to buy a rifle that already is set up for what you intend to use it for.
Your best bet.
After ingesting all this over the last couple days, sounds like you may be totally new to this game of long ranger type shooting.
Setting up a 1000y rifle is not for the novice, there are quite a few tricks that must be employed, that I would, here I go again, assume, that you are not privy to...right or wrong? I ask.
The cheapest, quickest and most effective way to break into the 1000y stuff is to learn to shoot first....Now, I'm not saying you don't know how to shoot, but there are some things one needs to polish up on before he starts lobbing lead to 1000y, if you want to hit anything down there.
You know, shooting quarter sized targets at 100y with a 22lr will go a long way in helping the 1000y score!
But, if you insist on the long range centerfire rout, then you might look for a good used rig of ????? flavor. Much cheaper than building one from scratch, your in the game instantly, all the bugs are already worked out of the rifle for you and you can concentrate on that little iddy biddy target way down there!
You don't specify 'what' kind of 1000y rifle your interested in, bench, sniper, palma, F-class, ect......
The rifle you currently have is an excellent rifle for its intended purpose, hunting.
I might keep that rifle for my deer slayer and practice some of the longer shots with it, you do not need a full blown 1000y rifle to learn on, kind like learning to fly from the start in a Lear jet, although I had a friend that did do that, but she had more money than the king on England!
Practice out to 500y with your .243, by the time you get done with all the reading and research on long range work, and by the time you can consistently put em; on target at 500y with that 243, you may have enough cash saved up to get yourself a nice, used 1000y rifle, and I'm talking with some pretty good glass on top....already set up and ready to go!
I don't think anyone here is intentionally trying to be disrespectful concerning the leapers optic you currently enjoy, but truth is, they are not all that, so like I say, plan on saving some cash and start practicing.