Welcome to B S.
I have a REM 7mm WSM. I 'm looking at overhaul much like your REM 300 WSM can you give me some guidance on this.
Toolbox....all I can tell you are my opinions and personal experiences. You'll have to determine what's best for you. The 7WSM is a excellent long range caliber and is effective to 1000yds and beyond with the right shooter behind the trigger. What is your budget for this over-haul? What will this gun be used for primarily? There are lots of variables to think about here that will play a role in determining your stock and optic selection. However, for shits ang giggles let's just say that it's going to be a dedicated long range rifle for punching steel at the range and maybe some long range hunting situations, maybe some competition.
First let me say, the rifle will probably shoot just fine as it it. Any modifications to the stock, optic, or cosmetics is obviously not going to increase the accuracy, or lack of that the rifle already possesses. These modifications are really more for shooter comfort.
Stock: Either buy after-market or keep what you have. I'm a fan of McMillian stocks, but they can be considered pricey to some. I've learned from experience in the shooting game that as with most things, you get what you pay for. The McM A5 is a great stock, or any of their variations. I also like the McM HTG for a all-purpose hunting/LR rifle. You can also look at the HS Precision and Bell & Carlson lines of stock. They both have lots of good reviews. They both offer full length aluminum bedding blocks in their stocks. They are all durable, for most purposes for the average shooter. You aren't going to be beating them through the rocks or running over them with vehicles. The McM is a proven battlefield/duty/service stock. People trust their lives on it so naturally your gonna pay more for the name and quality. If you have the chance get behind some different stocks and see what you like. ****Most importantly-buy used!!!! Get on SnipersHide or any of the forums and buy a good used stock and save hundreds as well as no wait time for a McM. There are tons of stocks for sale right now used on the hide. Most are lightly if at all used. If you must buy new, check Stocky's Stocks for the best deals. Also, any stock you get, it's a good idea to have it atleast skim bedded. You can do this yourself if your confident in your abilitys, or take it to a smith. Having it bedded is one thing that (can) aid in accuracy.
Optic.....Nothing I hate more than seeing someone spend $1000-$2000 on their rifle and then they stick a $150 scope on top of it with shitty rings and bases. Your automatically handicapping your rifle and making the weak link your optic. Buy once, cry once. Most people will tell you to budget as much for glass as you did for the rifle. For someone just starting out, I dont feel this is a must. I'm a NightForce fan, but they aren't cheap. They are rock solid reliable, built like a tank, able to take abuse, and have a good warranty where an English speaking person will answer the phone when you call the 1-800 number. I also really like the Vortex Viper line of optics, NOT the PST. I have a 2 vipers and both have performed well wiu no issues and both habe great glass. The Vortex warranty is also very reputable. There are really so many options to choose from for optics. Some advice though-buy an optic with matching turrets and recticle. Dont buy with MOA turret adjustments and a Mil based recticle. Get them matching, either MOA/MOA or Mil/Mil, whatever you prefer. This will help you down the road with ranging your targets with only your optic and also with making adjustments on follow up shots after a miss. Get plenty of magnification....the more the better out at 800-1000yds. 20x-25x on the top end is good. ***AGAIN BUY USED!!!*** SAVE $$$$
Rings/bases: NightForce, Badger Ord, Seekins, are all quality rings/bases. The EGW line is ok as well, but again you get what you pay for. With your base, make sure you get a 20MOA cant built in the base is this is gonna be a dedicated long range rifle. With a good optic, you'll still be able to get a 100yd zero and dial out to 1000yds with the 20MOA base. Don't go cheap here! This is an important piece of your equipment here. It's like having a 1000 horsepower race car and putting plastic lug nuts and cheap tires on it.
Trigger: your trigger may be just fine as it is. Typically a good 2lb-2.5lb pull is plenty.....most prefer less. Depending on what rifle you have, you may be able to adjust your trigger externally (40x trigger) or you may have to remove it from the stock. If you choose to replace it, the Jewel triggers are great. Rifle Basix and Timmney triggers are good. Most decent smiths can make a factory trigger just as good as any.
I think I've covered the basics for you, enough to get you looking and get your wallet puckered up. I can't stress enough though to buy used!! I don't buy any equipment new unless I just can't find it used on the Hide or other forums. Save money and use it for more ammo and gas!! If you have more questions just let me know.