Thanks. The cost of the Mansion is 50,000 Gold, right? If so, how much Gold should I have before I make the early mansion purchase?
First, check your expenses at the end of the day. Income is rather random, so it's better if you have enough money to fund a whole day of expenses.
Next, I think having 1-2 thousand gold in reserve for any emergency reconstruction you might need is fine. This is not a game that needs much emergency construction.
Anything else depends on what expenses you foresee in the future.
Alternatively, if you have a lot of tentacle eggs stockpiled, you can use them instead of reserve funds. At 5000 per egg, just one should cover any unplanned expenses.
Also, when the deadline for getting the harem endings are 50 days, does that include max affection AND brainwashing? Or just maxing out their affection?
Harem endings do not require any brainwashing.
But if you have a big enough stock of tentacle eggs (and you probably do, unless you deliberately avoid building tentacle rooms), brainwashing can be done very quickly, by giving the eggs to the demoness for mana.
On a similar note, you said in the guide to install every mod except the Complex Breeding Mod, right? If so, which folder do I put the mods in my DLsite copy of the game?
No, my guide is intended for unmodded game.
Complex Breeding Mod seriously changes game balance, even with "CB for dummies", but most other mods should not affect the strategy involved much. Those that do, are pretty much "cheat" mods, like a mod that makes slaves never become infertile.
The mod pack linked in the starting post of this thread, is meant to be added right into the game's installation directory. If in doubt, note that their language file should overwrite your game's language file, adding all the words the mods need to work.
I am not familiar with workings of DLsite. I either get my games on Steam, or pirate them.