Help with rear derailleur.
 

Hey guys, I have a Trek 1.1, it has the lowest line Shimano components. I would like to replace the rear derailleur to the Shimano 105, but when I look at different web sites there is several different 105 models. I'm kinda confused on which to get. Second question, if I change out the rear der to the 105, will I have to change the shifter to the 105 shifter also? Third question, am I better if just buying all new 105 components for the entire bike? Of course if it's not to expensive. Please share any helpful info.

And the rear cog has 8 different sprockets. So I'm assuming that means it's a 8 speed bike. Thanks for any help!

The key things to look for on a Shimano rear derailleur are series to figure out if it is 8, 9, 10, or 11 speed and cage length to determine gear capacity. Since you say your Trek has an 8 speed cassette and bottom line parts I'm guessing Shimano 2300. The big question is whether you have a double or triple crank since that affects the cage length needed, doubles typically use a short cage, triples use a mid length cage, the exception is doubles with very wide range cassettes with 30 or 32 tooth cogs which need a mid cage.
Current 105 10 speed is 5700 series and you may still find the occasional 5600 series part, either of these will work with your 2300 shifters, i would avoind the new 5800 series 11 speed part since I think its cable pull ratio is different. If you change to a 105 10 speed shifter then you also need a 10 speed cassette.

The variations in 105 are either generation/cosmetics, or the cage length ie. short or medium (I don' think there's a long, but maybe), or possibly a special version of mounting system.

So, if your bike has a normal gear hanger built in then the standard RD is right. You still have to decide on cage length, based on the gear range. I don't memorize Shimano specs, but a typical road setup with 39/53 and 13-26 needs 37 teeth (14+13) of take up capacity and is the limit for short cage. More take up would want the medium cage.

And yes it should work with the existing Shimano controls. You don'[t need to change the front, since the front and rear derailleurs don't speak to each other anyway. As far as whether you should do everything, it's up to you, but I tend to be wary of trying to radically upgrade a cheap bike, and (without knowing) suggest keeping cheap bikes cheap ad plan B bikes and buying better if you want better. However, upgrading part by part as parts need replacement is a different story and is never a bad move (within reason).

The front crank only has two sprockets. One smaller one and one bigger one. To shift gears, I have to push the the brake leaver to the left to get a bigger cog and pushing a small button for a smaller cog, ( I'm talking about shifting the rear der of course).