It can be really simple or really unbelievably complex. Totally up to you.

I think it's better to plan the project, buy the liner and then dig the hole. If you dig the hole and then order the liner that's more time for rain to fill the hole and make a mess. It does take attention to detail. You can order a liner and then decide to start changing the hole unless you're really careful. The other issue is laying out a 100'x50' pond 8' deep, ordering the liner and then start digging by hand...there's going to be a reality check pretty fast. But I don't think there's any right or wrong way.

I would not bother trying to use the pre-form as part of the new pond. It's a pain and not really worth it.

The size of the pump will depend on how big the new pond is going to be and more importantly what you want. Lots of people have little rules like the pond should be turned over once an hour, meaning a 1000 gal pond should have a 1000 GPH pump. But these little nuggets are just completely made up and repeated over and over and have no basis at all in reality.Ponds with huge fish loads need huge pumps. Water Gardens with a few don't need any pump. So in Water Gardens pump size is driven more by what the owner wants. UV filter, other filters, waterfalls, stream, multiple pools all drive pump size selection.

Since you're on a budget I suggest considering a multiple pool pond. You can add each pool as you can. Means you don't have to cut corners.

I suggest making a collar out of bond beam concrete blocks. A little extra cost but it makes building the pond easier, greatly reduces the chance of future leaks and keep the pond edges firm for years and years. Just dry lay the blocks on the dug out dirt. Some granite dust makes leveling blocks easier. You can use a water level to make all the blocks level. After the liner is laid the edges can be tucked into the bond beam blocks (unlike regular blocks) and then filled with soil. That leaves you only about 2" of exposed liner. That can be hidden with plants or very few rocks.

I like to build my own skimmer. Skimmers are nice but this also gives me a place for a submerged pump, behind the skimmer basket. That way the skimmer acts as a great pre-filter and access to the pump is easy.

I also like to make a catch basin under any waterfalls.