First off, welcome to the .338 Lapua Magnum bankruptcy club.
Below is my .338LM Saga for what it is worth. My Saga began with me finding a Remington 700 MLR that I scored on a killer deal and thought that it would be cool to shoot (it is), and I had been wanting a big bore, extreme long range shooter for a while. I did not want to go the .50 caliber route because I was not wanting to deal with a boat anchor, I wanted something that I could carry. Additionally I like big magnum boomers. Then came optics, mount and rings, bipod and on and on. I figured that since I had saved a lot on the rifle I would splurge on all the other paraphernalia since I could still use all that if the rifle turned out to be a bust. It didn't. The rifle, while bought on the cheap, is a hell of lot more accurate than I am. If I do my part it easily shoots in the half MOA range and even if I screw it up is still easily a sub MOA shooter. A couple of boxes of factory ammo confirmed that. But that was just the beginning:
To feed it, factory ammo was not a viable option, I was going to have to reload. I went with the Redding Competition Bushing 3-Die Neck Sizer Set, with micrometer seater die, micrometer neck sizing die and body sizing die. Also got a Redding full length bushing sizing die, nice to have at times as you sometimes want to do a full length resize. Also got a set of Redding bushings in .364, .365 and .366 and the Redding competition shell holder set. Once you figure out what bushing you are going to use the most, then buy enough of that size for all your dies since having to take your dies apart all the time to switch out the bushing is a pain in the ass. The shell holder set is nice to have if you have to bump back the shoulder, it takes the guess work out of the process.
I have since got the Redding Competition shell holder sets for all of the precision rifle calibers that I shoot, and while I have not used them much I like having the option to use them for shoulder bumps and not having to fiddle with die settings to get the bump right. I love the micrometer bullet seater die, never having to fiddle and fudge to get the bullet seating right is beyond words. Just dial it in and you are right on, don't know why I didn't go with this option years ago, but am now buying at least the micrometer seater dies for my other, most shot rifles cartridges. More dollars spent to upgrade tools but everyone knows that we don't reload to save money, do we?
As far as I am concerned the only way to go is with Lapua brass but finding once fired for sale is well-nigh impossible, some of my brass is on the 5th firing and I have not seen any real issues so far. If you can find once fired Lapua brass for a good price then jump all over it (and then let me know so that I can get in on the action too). In all I think that I spent almost $900 on brass and close to $600 on dies, shell holders, bushings etc. but I do not regret it. At anywhere from $5 to $6 dollars a round for factory ammo I believe that this is one of the few rounds where I am actually now saving money on the ammo by reloading.
If you are not in a hurry to get the stuff to reload for the .338 LM watch for sales on the brass and deals on the dies and other sundries. I bought my dies and stuff when Midway was still doing really good coupon codes and bought with a bunch of other stuff that I needed to get the maximum off deal.
I love my .338LM, it is a great shooter and is now my primary shooter for precision. Downsides are components. Primers have not been an issue since got Federal 215M's in quantity at the same time that I bought my brass. Sierra match king bullets in 500 round boxes are pretty available. The Lapua Scenar is a great bullet but I find that the SMK is more available and more likely to be found on sale. I, and the gun, seem to prefer the 250 grain but that is up to you, feel free to experiment with all of them. Powder is the most problematic component to find right now and at close to 100 grains a load goes pretty fast. Buy in 8 pound kegs if you can find it and buy in quantity to offset all the fees associated with powder, you'll need it anyway to feed the beast. Ramshot Magnum is now my go-to powder for all my magnum rifles. It meters well and I have found it be a nice, consistent and pretty clean burning powder. I switched to Ramshot powders for pretty much all my reloading back in the '09 scare/shortage since it was one of the only powders that was regularly available at that time. But now, alas, it is also in short supply.
I will say that I was not prepared for the cost of shooting the .338LM when I bought the rifle. Because of the gun I have since upgraded a lot of reloading tools to load better ammo for it, things like a Chargemaster, better case trimmer, better hand priming tool, RCBS case prep center, stainless media tumbler set up, etc. etc. ad nauseam. The list is pretty long, but these are upgrades that I have been meaning to do for a long time anyway, this just forced me to do it sooner and faster than I would have normally wanted. Be prepared for the huge sucking sound of dollars going bye-bye to feed the damned thing.
Oh, one last thing. If you are married and she is not into shooting, she will hate the thing, so you will have spend more dollars keeping her happy. Just Sayin'...