I have started my first 3.5 campaign and had the idea that as the GM I could still enjoy playing by making a character using the normal rules and running it as the villain. It would gain the same experience points as the party and level accordingly. 

It would not be a part of the party and would not travel with them. It would show up at points in the story, first time being in the first dungeon by killing the boss and replacing it with his Eidolon. I have been told it is normal to make NPC like this in D&D but in my experience GMing other games you could never make an enemy using the same rules as a PC. 

The concept is a summoner/paladin that is trying to save the plane but but has become more and more acceptant that the ends justify the means and has fallen form being a paladin but has not noticed as it was gradual. The players will see his evil actions and eventually decide to take him out, learn the truth and finish his quest in a less blood soaked manner. 

My idea was to give villain the same exp as the party, not take a cut but juts the same as what the leader gets form each encounter. This way I can enjoy growing a character like everyone else. 

I was planing to play it completely straight so no GM fudging for my character, if I get a bad roll I get a bad roll, no cheating. 

What problems would I have playing a straight character like this? How would I balance it against the party as an effective BBEG? For example, what starting level would people advise for a summoner/paladin against 4-5 level one players? I want him to last at least 2 encounters and be a fun challenge.