Things you can go budget on-
Water bottles (as long as you can drink from them and they fit your bottle cage)
Bottle cages- plastic or metal is fine
Cycle top- as long as they can wick sweat and have back pockets, you don't have to pay top dollar.
Socks- any thin sport socks will do
Cycle glasses- sunglasses (that are robust and can survive failing) are OK or go and buy safety glasses from a safety equipment store.
Underseat bag- as long as it fits all your gear of course.
Clipless pedals- buy a well known brand but you don't need carbon fibre or titiamum.
Cycle gloves- as long as they are comfortable
Helmet- as long as it is safety certifed and fits your head you don't need a $200 helmet.
Cycle computer- you probably only need speed, distance and maybe cadence at this time. Use you phone for GPS if you need it.

Buy the best you can-
Bike
Cycle shorts- too cheap and they will not last, pill, snag or be very uncomfortable.
Floor pump- don't buy the cheapest. Metal is better than plastic.
Cycle shoes- buy what is comfortable. Try not to buy full plastic or nylon sole shoes. Stiffness is good.

I think this covers it all. Have I forgotten anything?