2002 Mustang V6- Needs work! Modifications/Help

Hey guys, I'm kinda new to the forum, so please bare with me! I bought my first stang in 2007, White 2002 V6, I cant remember much from when I first bought it, but I got it with about 125K miles on it, and after doing a few things to it, it ran like a dream and was pretty fast, since then the power has been decreasing, The tranny fluid and rear axle fluid hasn't been changed, please ask me questions so that you can better help me! The car now is very near to its 150K mark (2200) miles, I missed my 3000K oil change which I plan on doing this weekend hopefully (Put seafoam in the crankcase today, drain it so it gets all the sludge out, only concern is I have a leak at my rear main seal, which is gonna cost to fix, so now im only measuring the oil and I have put a thicker oil weight) so I will go by points but please help me! I can't see my baby die!

2002 V6
-Near 150K
-All stock- New duralast battery last year, My alternator seems bad so I am changing that soon (And suggestions?)
-I want to get a cold air intake (not trying to spend the $230 on the K&N but whats the next best? or somethin on a budget
-I know im not trying to spend much and the more I spend the faster the car will be, I am not going to race it, dyno or anything like that, I just want it to look good, feel good, and drive good, also to last
-If I do decide to sell it, I want it to be easy to sell, although I want it to last so I need to make that happen
-Don't think I want to do the exhaust or anything, maybe add a turbo or something, as you can see I'm basically at square 1.
-Im good with working on cars and I know a lot of shops that specialize in mustangs but before I go in to any, cause I do most of the work myself, I wanted to see what needed to be done.
-Please go into detail with everything that needs to be changed, (fuel filter, o2 sensors etc.)

Thanks,

Again please bear with me!!

Yea definitely going to keep it nice looking, planning on dark tint, redoing the paint ( its white, its prolly cheaper to repaint it to white, but I'd love black, plus probably easier to sell down the road if its the stock color, dont you think?) gunmetal or black rims, I have the LED taillights, the LED third is on the way, the reflectors soon going to be smoke or clear with the headlights and fogs..

But I'm concerned now with the performance/reliability of the engine now, its getting close to its 150,000th mile, and I want to get everything changed thats affordable that will be best, obviously I don't want to replace the whole engine! But o2 sensors, stuff like that that needs to be changed, im not entirely sure.