A lv3 forge is better than a lv1 sub so if you want a sub make sure it is lv2 for better plunder. Find some people that will let you steal from them and attack osfs to get the most money possible.
I believe he said pure spy not hansel build? Anyways to be a pure spy make sure you have lc'd as you will upgrade very slowly. Here's a good way to look at it, whateer strength you're at when you switch to pure spy, you'll be that strength forever. If you want lvl 3 guilds you can probably get away with just selling all your allies (if u have MP) to buy the guilds if u want lvl 4 you'll need 21.6b banked pure spies make very little money and are a good support build so choose wisely *note* never reset to change to pure spy
I made to conversion as well, early on in the game. So i have a fair amoubt if experience with the build. I dont know the plunder difference bc i switched early, but at 100k spy attack, stealing from LC's gets me about 4 mil. Banking your money is key. And you have to tinker with how many steals with full regen gets you max amount of gold. Its reccomended you have a forge with allies for OSF hitting, because you make much more gold.
Its pretty cut throat simple. Just reduced plunder unless you go Hansel build, and get lvl 4 guilds once you get all lvl 3 guilds until you hit 900mil land. Different warfare tactics but im sure you'll figure those out.