Well, it is your car... do what makes you happy, what works for you. Who cares what people think of your vehicle? Like my white stock auto would never, ever, make it into magazines like Hot 4s or Autosalon. But, it is legal, insurable, is cheap to run, drivable on dirt roads, and as of tomorrow, paid for. I have made some minor asthetic improvements by way of colour-coding and I have upgraded the stereo a bit because I like my music. These are the things that are important to me.
What is acceptable? Depends on the person. From my point of view, looking thru various modified 4cyl magazines, the cars are all cookie cutter jobs - lowered, turbo, huge rims, new paint and interior, some subs, new bodykit. They all look the same, it is very boring. Build one of these and there will be no further question as to your lack of imagination.
If I was going to do up my '05 I would drop it a little (would instal a pedders sportsryder suspension), have 17x7" wheels w/215/45 R17 rubber. Then I'd upholster the interior like that of a BMW - recolour the dashboard black, maybe cream leather on seats, door trims etc. Then I'd install maybe a sat-nav/stereo head unit. Engine wise, maybe a small low-boost turbo to aid my progress. This is my corolla Utopia, a true mini-lexus.
But just doing the sums now it'd cost upwards of A$15,000, for a car worth A$19,000 or so. After the upgrades, the value would be... still $19,000 (mods usually don't add a jot to the value). Bear in mind your car is a depreciating asset; it would make more sense to prolong the experience rather than enhance it and spend your hard earned on upkeep.
Again, it depends on your personality, finances, time/effort prepared to put in - not what some other guy thinks