Can you guys recommend me some weapon mods where the weapons are balanced,fit in with the game and spawn on enemies and in vendors inventories?

to name a few

20th century weapons
Zealot's ACR
Vltor SRB
FOOK2 - adds masses of weapons, though rebalances gameplay
FWE - same as above
Brupo's M1119

there are plenty of others but those are off the top of my head and range from pistols -> machine guns

http://www.fallout3nexus.com/downloads/file.php?id=3388

I would install FWE but there are some changes they make in that mod that I don't like and they can't be turned off.

Also some of those weapon mods don't really fit in with the fallout setting.

A good example of the kind of mods I'm looking for is this

http://www.fallout3nexus.com/downloads/file.php?id=4258

easiest and fastest thing to do then would be to look through the weapons at the nexus and decide which are right for you. there are a ton of classic weapons packs, although i dont use them so i cant give you the names right off the bat

If you want lore fitting weapons, look no further than the http://www.fallout3nexus.com/downloads/file.php?id=2307. It's that mod where FWE's guns come from (AFAIK).

Are they balanced though? Because I heard that they're not.

Actually I have another question, is it possible to take one of the mods where they only add one weapon to one location and changed it so that it spawns on enemies and in the inventories of vendors?

Yup. It takes a bit of work, but it's completely possible.
Personally, I find that 19th and 20th Century Weapons is well balanced, and since it does allow for the weapons to show up in the hands of raiders, slavers, and pretty much everyone else, I'd say it's well balanced.

How hard is it to make those changes?

Well, you need to place them in leveled lists. Other then that, not very hard. Just gets tedious placing hundreds of weapons in several lists, but that only happens if the mod adds hundreds of weapons and you want them in many lists(so they aren't as rare).
I'd guess that you're adding a few weapons to a smaller amount of lists(as to make getting the weapon a treat), so it's not that hard. Just time consuming, really.
EDIT: Oh, and while the rule of last mod wins does apply to leveled lists, this can be avoided by either building a merged patch, or by using Gary Bash and making a Bash Patch, which helps even more.

The vast majority of FOOK2's features are optional and/or configurable through the menu, hopefully aiding its ...elasticity for the user. It also, of course, adds http://i196.photobucket.com/albums/aa150/JustinCoherent/Fallout%203/FOOK2WeaponsALL.png and other goodies though, so I'd suggest checking it out at some point.

I have to agree with JustinOther. It also stands to say he should know all about Fook 2. He created it.
Fook 2 can provide you with good guns too.

Also, please ignore any display errors on automatic shotguns in any mod, Fallout 3 is bugged in their display.
That's the only balance issues I heard with gun mods.

FOOK was created by numerous modders, so I can't take credit where not due. I merely helped create it and am just part of a collective or team, doing most of the merging/scripting/menu related stuff. Without the generous contributions from all of said other modders and all of the blood, sweat, and tears shed by the FOOK Team, FOOK would be nowhere near as *massive or diverse as it has become. Thanks for your enthusiasm, DavidB1111, just the same.

*FOOK2 is 'bigger' than 3/5 official DLC's with a cumulative record count of about 44,000.

Wow FOOK 2 looks incredable, to I'm not the kind of person to download mods that don't fit in with the lore.

Okay what program would I use to do that.

Either FO3Edit or GECK, both of which are must haves for modders.

Which is easier?

Also can I get an answer to this? I looks interesting.