I agree with Andy. It's many years since I entered any IPMS comps, and some of the rules have changed since then, obviously, due to items such as PE and resin detail parts, which hadn't even been thought of back then. But, there are still variations of the 'Out of the box' standard, as far as I know, so you don't have to throw a shed load of after market parts at a model to make it good, and a possible class winner.
The main thing is, overcoming the impatience - as Wojtek often states, modelling is not a race, and a build will take as long as it takes. Rushing to get a model finished is just rushing to lose points, when, for the sake of a few extra minutes here, or an hour there, a 'so so' model can be made into a winner and, what's more important, or at least to me anyway, the satisfaction of knowing that you like the model, and it will pass close scrutiny, instead of knowing, deep down, that it could have been 'X' percent better if more time and concentration had been invested.
Have a look at the rules Andy posted, get an idea of what's required, and then plan and execute the build in logical stages, taking your time to ensure things are done to the best of your ability, not the pre-made work of an after market 'add on' and, above all, enjoy it - it's a hobby, and a creative one at that - if you really want to have something on the shelf a couple of hours after opening the box, then buy a ready-built or die-cast model, but if you want something good on the shelf that you created, then slow down, concentrate, and enjoy the finished product in all its glory.