Hi Budforce,
As you maybe already know, the only way to level up your Mastery Rank is to level up weapons and Warframes to level 30.
Like if you completed a mission using already maxed rank 30 weapons, Warframe and Sentinel/Companion, it would not add anything to your Mastery Rank.
So I'd recommend maxing anything you can get your hands on, all weapons are adequate at the start of the game anyway.
I kept ranking up everything till I got to MR14 (Aklex Prime MR requirement changed to MR15 after Feb update) as it was the max you needed to unlock all available weapons and Warframes so you can use them (if you have all the parts to craft them).
As for selling, yes you would have to level up again if you bought a new one and if you've put an Orokin Catalyst in it or Forma'd it you won't get them back when you sell it.
Some people have told me it's best to keep all your weapons as you never know if DE will buff it in the future and it then becomes the new meta weapon.
Having said that, a lot of weapons are cheap to make and their blueprints are easily available on the in-game market so you might as well sell them now and get them again later if need be (no point in keeping the MK1 versions).
Also check the Wiki to see if it's used in the crafting of another weapon, so might be worth hanging on to.
Although most items in the in-game market require plat to buy outright without the need to craft, there are a few that only require credits so look out for them when you are in the Arsenal under Equip, the ones with a credit symbol can be bought already crafted though you don't get a slot included like you do with the plat ones.
Hope this is of some use to you or to someone new to the game.
Cheers,
Edit: For the concise version see TheBrooder's reply.
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