Like most of your threads, your OP has many indicators such this project already seems beyond you. You have a fundamental lack of understanding, on multiple levels.
1) Custom Mausers were the rage before the Rem 700 was a gleam in someone's eye, and still a very popular option, since base mausers are still readily available. They aren't without headaches, you'll spend far more sporterizing one than starting with a more expensive action would cost. There are many reasons why custom Savage and Remington's have displaced Mausers as the most popular base, but the Mauser action remains to hold the title as the most commonly customized rifle in the world.
2) PT&G does offer short action K98 bottom metal. It's not hard to find on their site, and it does use the fantastic AI 308 mags. Anyone prepared with the knowledge to build a rifle would realize the 308win is not compatible with 300wm magazines, and would know only ONE of the three k98 action lengths would be compatible with the 300wm bottom metal (and vice versa for the 308 short action model - three lengths of action = three lengths of bottom metal). Since you're a kid trying to figure out custom builds on the cheap, I might point out here the fact the AICS mags tend to run around $80 each, which really eats into your "savings" by using a (poor choice) Mauser action for your tactical build. Even the "iffy" magpul AICS pattern mags are $40. There are other bottom metal options out there which utilize CETME or M14/M1a mags, giving you access to $10 mags instead of $80 mags... either being a bad choice, since the Mauser pick is ill advised.
3) Feeding reliability is one of the most difficult aspects of the Mauser DBM conversion. This isn't a bolt together option, experienced and knowledgeable smiths still sometimes don't get it right, even with premium parts, proper training, and years of hands on experience. Even Gibbs rifle company, some of the best rifle builders in the world, missed the mark with several of their conversions.
4) You do not need a flat top line to mount a rail. Fitting a rail to almost any action which is not top fed or top eject is really no different than fitting to any other - flat to round, offset height, round to round, flat to flat... it's all common, and all easy.
5) $1000 to build a custom Mauser? Yeah, ok. Wanna buy the Golden Gate Bridge? $350 for the action, $450 for a short chambered, turned, and threaded match barrel, $40 for the chambering reamer rental plus $50 for renting the pilots, arbor, and T handle plus cutting oil, $40 for the reamer rental, $150 to $200 for the cheapest of junk stocks, $50 for bedding compound and pillars, $150 for the bottom metal plus $40+ for one mag, $50+ for the mount... That's already over $1200 before you even get to ANY blue printing or trigger work, and before rings and the optic on top. You'll need to pay a smith to true the bolt lugs, then you'll be $500 into the hand tools required to true the receiver lugs, receiver face, chase and true threads, and true bolt face. And of course, that will do nothing to improve fitment of your lugs to the raceway, bolt to the action bore, or pin to the pin bore - if you find your action to be sloppy, sleeving is a remarkably expensive, and will overshadow the cost of your bargain barrel action. And of course, that offers no budget for action and barrel wrench, torque wrench, inletting tools, cost of bluing - especially if you take it on yourself - or any other tool you may need. If you just want to screw something together and pretend it's cool because it looks cool, great. If you want it to be as functional as it looks for its application, you're not getting it done for under $1000. Lots of us have been down the custom rifle road - many of us live on this road, actually having built what you're considering, and many others like it. If it were as simple and inexpensive as you're planning, more people would have done it, and those of us with dozens of custom rifles in our safes would have spent a lot less money a long time ago.
6) And maybe the most critical aspect to my string of comments - anyone can screw a barrel into a receiver, it's not so different than screwing in a lightbulb. Screwing in a barrel isn't gunsmithing. There's much more to barreling or rebarreling a rifle, as I've somewhat outlined above in the list of tools, than simply buying parts and screwing them all together. Bolt guns aren't AR-15's, especially Mausers.
If you DO want a custom build someone like yourself can accomplish without extensive tooling and on a low budget, build a Savage, or build a Rem 700 with a Remage barrel nut conversion. Plenty of "drop in" aftermarket parts, and specific designs which allow customization without precision tooling, while still yielding precision results downrange. Donor Savage actions can be found for $250, drop-in barrels readily bought for $300-400, plenty of stocks and DBM options, some even factory offered. Rem 700 ADL's can be bought new for $350, stocks, barrels, and bottom metal sold off for about $200, getting you into the action for $150-200, and there's no more popular action for aftermarket options. A Blueprinted 700 action can be bought from PT&G for around $450, adding a Remage barrel for another $450, Boyd's At-One or Pro-Varmint stock for $200-250 plus $50 for blocks and bedding, and a Seekins rail, you're all in for $1200 before optics and you'll have a rifle which will outshoot your "bargain" Mauser for several hundred dollars less.
