Just an open suggestion,
Get over to Brownell's and do some looking. There are a ton of options there to get you started off figuring out how/what you want to put together. There are also several other places which offer custom products built for the Rem 700 action. Just do plenty of searches and you will eventually find just what you want.
As for caliber, The shorter cases in the longer action will work with a blocked magazine. However usually these aren't detachable. Not a real biggie unless it is a deal killer. In most cases your not going to shoot more than 5 rounds anyway and a standard mag on the Rem 700 usually allow 4 easily with '06 based cases with one in the pipe.
Barrels, well there again, it a buyers choice and market. There are several really good makers and a few great ones, just depends on what your final decision is, and if the maker has what you want. Bumping up from the standard 700 tube to something like a Heavy Sporter would be fine without getting the added weight of a full bull barrel. I have a couple built with them and they are nice, not overly heavy, and they balance out real nice. My last build was a 26-06 AI and was built for LR and heavier than normal bullets. I went with a Broughton 5C 4.1 Mag Sporter, 1-9" which finished out at 28". It has worked out VERY well for me and is a breeze to clean up. When your looking you might consider one of the canted land type barrels. DOn't forget twist rates either. IF you go too slow you could give up accuracy with any heavier bullets you might want to shoot later on, if you go too fast, you could bump into your velocity due to getting higher pressures a bit quicker. Usually sticking with the standard twist rates will get you really close, but personally I prefer a 1-10 for 30 cal over the now predominant 1-12 versions. Not that I shoot that much of a heavier bullet, but I haven't found where the 1-10 in the half dozen or so 30 cal rifles I have, has been a downfall.
As for caliber, well if you stick with the parent case of 30-06 for the long action you have plenty of very fine factory type calibers, and several custom chamberings which are also VERY effective. Personally I wouldn't look into the Ackley cases unless your simply building something you REALLY want. Some are great, some not so much, but they are a bit more work to keep fed. I built mine due to I have an abundance of both bullets and cases which I wanted to use up in something and my standard 25 simply wasn't getting what I wanted done. The loads I use burn more powder than the original, but it is also a slower powder behind a heavier bullet with more barrel to burn it in. Simply put I am getting the velocity I wanted and NOT having to push the upper limits of the brass or action mainly from the longer barrel.
That said just a couple off the top of my head I might look into, first might be a 6.5-06 if thinking semi custom or a 280 in either a straight for bullet weights up to around 150grs, or possibly an AI version if looking at shooting the heavier bullets at longer ranges. The 6.5-06 or the 280 AI are mostly a handloading only type proposition to get the best out of what you have built. However with the AI you still have the option of shooting factory rounds initially for barrel break in and hunting, then also having the formed cases for loads. Don't think the standard rounds in the AI chamber can't be accurate, I have several different AI calibers and they will all shoot the factory bullets into 1" or less groups. In fact the one .223AI Contender barrel I have has only had about 100 handloads through it compared to 6-700 factory rounds. I simply quit trying to load for it as the cheap factory stuff shoots so well.
For stocks, well there again it will depend on what you have in your mind. There are some really nice ones out there depending on just what your intended use will be. I picked the HS Precision M24 A1 for my 25-06 as I wanted it to be close to another rifle I already had. I also liked the two swivel studs on the fore end which allow either two different mounting positions for my bi-pod or I can sling it while the pod is in place and not have the added tension applied to the pod itself. I would have gone with the McMillan of similar design, but they were backordered out to around 18 months at the time. No matter the brand or type I do suggest looking at ones with the full aluminum or at the very least pillow blacked types. Then when your ready to put it all together, have your smith skim bed the action into the stock. This will give you the best of both worlds and take out all or most possibilities of anything moving or slipping on down the road.
Lastly, take your time, and pick out the things you will be happy with. Even if it takes you a year or more to gather them all up it will be something your going to like to shoot, and will be what you want when your done. I think it took me around 18 months to finally get all of my pieces together for my 25-06 AI, but I have been VERY pleased with the results from day one. With factory rounds it will easily keep 1" at 200yds, and with the loads I have worked up for it, in calm conditions 1" at 300 is pretty easily done if I am on top of my game. Combine that accuracy with a 120 or 130gr bullet leaving the muzzle between 3250 and 3350fps, and you can see where I could easily have a good time on the hogs, yotes, or anything else I wanted to reach out and touch.
Good luck with your build.
