1. sensors - i will use Analog and digital sensors.
OK. Any Arduino will work. You can then read as many of these mysterious sensors as you have pins.
2. SNMP protocol- i will use Ethernet (RJ-45 connector).
You can make GET and POST requests. If you make an appropriate GET request to an appropriate site, that can result in e-mail being sent. The Ethernet shield doesn't let the Arduino BE a mail server.
3. LCD- will show until 5 values.
So, get one.
4. Arduino type- i want one that will manage all sensors.
So, get one. Since you won't say how many of each type you are thinking of, you get to do the research. Since you are planning to use an ethernet shield, the Mini, the FIO and other non-shield shaped Arduinos won't be useful.
A. can Arduino sent snmp trap or just know how to work with snmp set/get ?
B. can i change some on the analog sensors values remotely (to connect from the Ethernet and to change them ) ?
You can't. Analog sensors measure things. You can't change how they measure things. The manufactured designed how they work. You can't change that.