sounds like a fun project. I have a reef and in the process of setting up a 40br planted tank...
you have lots of options in tank size, filter options lighting heck you name it. IMHO I would kinda chill out a bit and really really think about what you want reef or planted size etc. join some local clubs ppl are always selling or giving tanks you can find some crazy deals...I once gave away a 240 gal tank with new reef lighting as I moved and had to have it gone.
also in your budget I did not see stand. but looks like you are going the DIY one? that will still cost around 100 or so, still a hell of alot cheaper than buying new tho.
for sizing a like longer than higher. the 6' 125 IMHO are better the the 5' 150 but it all comes down to taste.
if you are looking for a nice rimless low iron tank check out SCA aquariums:
they are coming out with a new 150 gal. I believe the price is around $800 =really great price ( look on FB) I have there 90 gal. 12mm and I can say there are very nice tanks.( was the largest water volume is the smallest foot print I could fit.) the overflow box is a PITA but could be made with out or and the very least removed. just an idea
as much as I enjoy the sound of HOB filters I also would go with a canister or sump plumbing is really not that big of a deal + it gives you room for heaters, plant nursery ,fish time out zone, etc. ( depending on how you make it )
another option is make a DIY bucket filter like joey, king of DIY on youtube. if I was setting up a bigger tank I would go sump or bucket filter...
for substrate find a tractor supply store and grab a few bags of black diamond blasting (BDBS) sand 50# under $10 I found one 45 min, away and got 2 bags for my 40br in the works. you can cap dirt or go all sand and use roots tabs for the plants same some $$ there it looks great and lots of ppl use it.
if you get up to boulder or the higher mountain area I am sure you could find some driftwood. take the wife on a romantic picnic and oh look some driftwood and cool rocks might as well take a bunch home
just some random ideas and thoughts, be sure to do a build thread!