Newbie! many questions...

Berger stuff is just pricey. Mind you, any match grade bullet will be pricey. Hornady makes a 140 grain Match bullet with an MSRP of $37.77 per 100. Sierra makes a 140 and a 142 grain(dunno snicker) Matchking with an MSRP of $41 per 100 and $46.18 per 100(needs a 1 in 8 twist) respectively. MSRP doesn't always mean retail though. $34.99 per 100 for Hornady ELD-Match bullets in Cabela's. $46.99 per 100 for Berger VLD Match bullets. Ya gotta shop around and try 'em all in your rifle.
You will lose velocity with a 20" barrel and the rifle and cartridge are hunting things(that they excel at doing) more than target shooting. Doesn't really matter as it's what you have. Go shoot and see if you like the target shooting game. Then think about a pure target rifle(like a 6.5 Creedmore) when money allows.
However, a 140 sighted in at 200 yards(2.4" high at 100) will drop 57.8" at 500 yards out of a 24" barrel. It is suggested that out of a 20" barrel go for around 2700 fps(which is not done with match dies with a seating depth micrometer or any of the other gizmos available. It's done with meticulous loading techniques starting with weighing every powder charge.), but those are max loads with some powders only.
The shell holder has nothing whatever to do with accuracy either.
A wind meter next to your head won't tell you what the wind is doing at 100, never mind 600. The range should have flags every 100 yards. You need to learn how to read 'em. And read the mirage. Think the heat waves you see coming off pavement in summer. Wherever Cow Hamsher is.

Berger for matches. Sierra or Hornady for practice. You need more bullets, certainly.

Lapua brass, it pays for itself over time. You need more brass.

Wind at the shooter's position is much more critical than any other position down range. Kestrel are the ticket for a comprehensive and accurate solution (remember, your Applied Ballistics app works with station pressure, not barometric pressure). Litz and Doc put out an article to this effect recently explicating the difference in drift effect by a shooter position wind vs. a target position wind. Think about a bowling ball rolling towards pins - how far off target will it be if it gets a slight bump as soon as it hits the lane, compared to the same slight bump right before it hits the pins? An early bump yields a much farther dispersion than a late bump.

20" barrel will be fine until you burn it out, then replace with a longer tube to pick up some speed. The Swede doesn't have the best barrel life, you could go through 2 barrels a year if you were shooting a heavy match schedule - you'll see it start opening up around 2,000rnds most likely. How long it takes you to shoot that many will dictate your chronological life. A 20" 6.5 Swede will have plenty of speed to get to 600, or even 1,000yrds.

The Stoney Point, now Hornady OAL gauge system is useful, but not overly valuable if you're only focusing upon one or two bullets for one or two cartridges. I have a tutorial (which I owe to a few other folks on here) on how to use them effectively. You'll need the headspace comparator, the bullet comparator, modified case, and OAL gauge spindle, as all of the tools have to be used in congress to be of use. Smoking a long seated bullet will tell you everything you need to know about your throat and leade, as accurately as the Hornady system can.

Shell holders are shell holders - take your pick.

Get a good tripod for your spotting scope - and better still - get a smart phone adapter to mount on the spotter. Sure saves on eye strain to just peek at the phone instead of needing to focus into the eyepiece.

With Lapua brass and a factory chamber, neck turning won't be worth the investment.

Lots of options for neck turners. Lathe type is the nicest option for processing speed, the RCBS reamer/cutter heads for the TrimPro work fine. The Forster hand neck turner is better. The 21st Century lathe type is the top line.

Those 3, plus maybe the Hornady lathe type as a replacement to the RCBS, are the only 3 for which I would spend money.

But again - if you are using quality Lapua brass in a factory barrel, don't bother with neck turning. When you burn out that barrel and order a custom spec'd chamber for the bullet you're shooting, then you can worry about neck turning.

What is the specific competition discipline? Precision Rifle is a bit different from F-class is a bit different from bench rest. It's fine to be in multiple disciplines, but it helps to know when advising.

Depending on your constraints, you may be able to shoot a lot more with a less expensive bullet like the 147 Hornady ELD. If you can't afford to shoot 500-1000 rounds per year, your marksmanship and wind reading will be negatively impacted. Better to choose a cheaper bullet that you can learn more with (by shooting more) than a pricier bullet that may allows (slightly) higher scores once you have more experience.