[GriefPrevention does not protect listed IDs]

Try BanItem or TekkitRestrict since they were made for blocking mod items.

[Achievements keep resetting]

Are you playing Tekkit Classic to get the achievements and then playing vanilla Minecraft after? I found that this sometimes causes the problem, But not always.

[Best Protection Plugin?]

Before I tell you, All I can say is to get rid of LWC since people hate it when they can't steal from other people. Also probably the best and most familiar way for users to do this is factions. I got the correct version to work with Tekkit Classic.

http://dev.bukkit.org/bukkit-plugins/factions/files/38-1-7-5-beta/

[Help Making Server Plugin - Better Hack Detection]

This plugin would take ages for just you to make, So I would try spread the message on other places since if there were coders on here, Willing to help, They would probably want some money or something more than just credit.

[Looking for a plugin]

1. Nice idea for a plugin, But I don't think this has been made.
2. Only chance of getting it is to make it yourself (Shouldn't take too long if you know enough java)

3. If option 2 is out of the question, Then find someone to make it for you. You probably will have to pay some money this way.

[FPS lagg]

Tekkit Classic is very old now and stuff like this is expected to happen, My brother's computer is a beast and he will gain massive lag spikes from all the old mods, etc.

[problems allowing players to join my personally hosted server]

I've had this problem aswell, But fixed in no time. I'll send a copy of my server.properties file incase that helps. (I'll block out anything personal or advertising stuff.)
 

#Minecraft server properties

#Mon Oct 06 21:44:32 BST 2014

allow-nether=true

level-name=world

enable-query=true

allow-flight=true

server-port=*****

level-type=DEFAULT

enable-rcon=false

level-seed=

server-ip=192.168.0.** (Open cmd and type "ipconfig" to find this, It will most likely be at the top of the text)

max-build-height=256

spawn-npcs=true

white-list=false

spawn-animals=true

online-mode=true

pvp=true

difficulty=1

gamemode=0

max-players=32

spawn-monsters=true

generate-structures=true

view-distance=10

motd=*** Tekkit Classic

 

If you're still having problems. http://portforward.com/english/routers/port_forwarding/