I have three concerns:
1: Do the mods make it crash? Should i patch it up before modding or will it run O.K even without that?
2. What are the best mods to get, for a person who role plays all the time?
3. Any mods i should avoid?

Yes, the rest of your statement is meaningless.


Questions far too general to provide meaningful answers.

The game is fairly buggy and will crash with or without mods. Chances are raised however with mods depending on how complex they are.

mods to avoid: http://tescosi.com/wiki/Popular_Outdated_Mods
this site has a lot, take your time.

Have you played Oblivion before? If not, it's best to just play the game for a bit, get an idea of what you want to change, and then go looking for mods to fix those things.

Start with the unofficial patches (you can find them on TESNexus) and go from there. Assuming you have the GOTY edition of Oblivion, it should be up to date, but the unofficial patches fix a lot of things Bethesda never got around to.
It's all too easy to make a crashy game, so make sure you read the readme for any mods you download, and make sure they are compatible.

Once you have a sense for what you want to change, Wrye Bash is the best tool for installing mods and http://oblivion.nexusmods.com/mods/35230 will show you how to use it. Oblivion will crash and do even stranger things if you neglect to read the readmes and just install stuff sight unseen. It will probably crash anyways, just not nearly as much.

Lots of people have lists of "essential" mods, but those lists are always different.
I like Uhttp://oblivion.nexusmods.com/mods/19370

Here are some lists of mods:
http://www.gamesas.com/topic/960829-environment-mod-list/
http://www.gamesas.com/topic/814477-magic-overhaul-comparison/
http://www.gamesas.com/topic/820948-scaling-overhaul-comparison/
http://www.gamesas.com/topic/1093197-running4covers-list-of-stealth-mods-2nd-thread/

As you might suppose, these sorts of questions get asked frequently. Take some time to look through the forum for previous threads, starting with the pinned ones at the top of this forum.
Some others you definitely should look at:
'http://tescosi.com/wiki/Main_Page',
'http://www.gamesas.com/topic/1232576-complete-modded-oblivion-installation-guide',
'http://www.gamesas.com/topic/1395654-new-to-mods',
'http://www.gamesas.com/topic/1368706-obmm-alternative-or-solution',
'http://sites.google.com/site/khettienna/recommended-oblivion-mods'.

Don't get too discouraged by the previous replies, but also you sound like you do not have realistic expectations in so far as modded Oblivion is concerned.

Yes, either way you will have some crashes to the desktop (CTDs). They are a part of the experience everyone has to live with, but thousands do live with them. Save early , save often, keep your old saves.

Re 1. You will need Oblivion patched up to v1.2.0.416, at a minimum. It is also strongly recommended to install the 'http://www.gamesas.com/topic/1385468-relz-unofficial-oblivion-patch-uop-v341', now at v3.4.2. If you have the Shivering Isles expansion, then you should also install the 'http://www.gamesas.com/topic/1398731-relz-unofficial-shivering-isles-patch-v151', v1.5.1. If you install any of the DownLoadable Content (DLCs), such as Knights of the Nine, Vile Lair, etc., then you should install the 'http://www.gamesas.com/topic/1399201-relz-unofficial-official-mods-patch-v17', v17, as well. These all fix known problems.

Re 2. Very subjective, and there are literally tens of thousands to choose from. What sort of characters do you like to play? Before you get into modding the game, definitely play 'vanilla' for a bit to see what you feel the need to change.

Re 3. This is more a matter of 'which in combination with what'. Overhauls are more likely to have conflicts with other mods that (perhaps more narrowly) try to alter the same things. But some of the most massive have patches to get them to work together. And it's often difficult to tell which mods conflict just from their descriptions. There are tools available to you that can do that, but they are better to wait on until you have a need and the vested interest in learning them.

Have some patience, you will get there.

-Dubious-