I would get what you have picked out first, hook them up, get familiar and enjoy them and save the sub and amp for when you can spend the money. they are the most expensive part of a system and not where you want to skimp... IMO...
In fact, I just had my stereo installed in my Jeep and while I was there, I was able to talk them in to giving me the last remaining Kenwood Excelon 10" subs for a steal. Appearently Kenwood is no longer going to make them, and they were what i wanted. So instead of 260 a piece, I got them for 240 for the pair.... So shop around a little too, deals are out there.
I would also caution you to buy these units at wal-mart. they are cheap cheap for a reason... a knock off brand like Fusion or Caos is going to give you issues. I had one from there and it only lasted me three weeks. Sony would not be bad, but not from wal-mart, they are knock-offs branded with the sony emblem in china and shipped here.
This is what I would recommend, if you are looking for a decent quality and still a good price, look out for sales at places like Hhgregg, or Bestbuy.... they are there, you just have to search. Or visit
www.crutchfield.com and search their "refurbished" or discounted items. Jensen, JVC, Pioneer, Alpine, Rockford Fosgate, and Kenwood are the brands I would look for. Sony will be a good competitor too if you shop outside wally world.
I am not a fan of over spending, but if there is one thing I have learned in my life is that money does buy quality. If you only spend 20 bucks on shoes, all you get is a 20 dollar pair of shoes... know what i mean? Its hard to explain sometimes, but trust me, in time you will appreciate that perhaps a set of speakers from Pioneer might be 40 dollars more per pair, but in the end, they will last longer, sound cleaner and treat you better.
However, I know this is more then you asked for an answer, and I do apologize, I just like to share what I have learned about shopping quality over price. My stereo head unit for example cost me 750 bucks, four years ago. since then it has been in six different vehicles, and it still kicks great! So take it from me, spend a little more(not nescesarily 750 bucks...lol... but like 120-180 for the head unit) and in the end you will be much happier with the results....
Anyways, as for the subs now vs later question, thats my answer. i would wait until you can get one or two that are clean sounding, hard hittng and worth every penny you spend.