Totally agree with Axolotyl about finding cheap stuff on ebay. I think that is good advice on anything GW related. Paying full retail for it is really silly. Unless you have a fair amount of disposable income and a really amazing local hobby shop that you game at frequently, then by all means.
As for substitute models as I said in the other post I have been doing that as well. The trouble is you won't find many of the newer 8th ed era models really cheap (unless you get super lucky). Salamanders don't usually go below 20 per model, the new big stuff rarely goes below 45. The fliers ... if you get the old metal ones you can sometimes get them as low as 10+shipping. Krox go for around 10 if you can get lucky. I actually picked up a metal carny for $25 (with 5 shipping) but some people might not like that model as the new one is much larger I think. You can get old style Saurus and skinks with bows super dirty cheap, etc. and some character models can be had for cheaper prices if you get lucky.
If you want bigger dinos though to approximate the new stuff I'd go with plastic dinos, and then bitz order some of the accessory parts (which you can usually do for less than 10 bucks). You end up spending say 25 or so per big model vs. 60 ... so its not a massive savings overall (save yourself like 75 bucks) but if you really want to try to do the project as cheap as you can its a good way to go. Like I said for me, I'm never going to do tournaments with my army so paying extra for the GW stuff isn't really that important for me, that said I do want models that look good and are generally WYSIWYG.
Post some pics of your stuff when you get it ... I plan to as well once its assmbled and primed and then when I'm done painting it!
Anyway good luck man!