Since we don't have any way to support auto-recurring payments like an installment plan, it sounds like you'd want to set up at least 3 products: the $895 "upfront in full", the $395 "down payment", and the $197 "installment". Then customers would click Add to Cart for the payment they want to make. You could use our "Variations which tell more..." setting to add a text field to the Add to Cart buttons, where the buyer could enter their domain name.
I didn't quite follow what you meant about having only 1 of each product for sale? What I described above would be "generic" products applicable to any Web site project; there's no reason I can think of why you'd need a completely separate product for each client? At any rate, you can enable Inventory Control with stock of 1 and also use Variations for that product. Consecutive product additions -- when you add a product and then proceed directly from the final button-codes screen for that product to add another product without returning to Seller Admin first -- will retain the last-added product's settings as a basis for the next product.
We only support setting affiliate commissions on a percentage basis, not a flat-amount basis, although you can manually grant specific affiliates an extra percentage or per-sale flat bonus amount in Seller Admin > View/Edit Affiliates.
In your Seller Admin > Affiliate Program Settings, you can set up a common (default) percentage for all products, and a Hop Link landing-page URL. You can just leave this at 0% if you don't want every product to grant at least some commission, and yet still setup the common Hop Link. Common percentage or not, the common Hop Link will allow affiliates to obtain code for a single referral link that tracks their commission for all/any of your products.
You can override the common default percentage for sales of any specific product(s) by going to Seller Admin > Product-specific Affiliate Programs, select a product and click Edit Affiliate Settings. You can set a percentage only for sales of that product without also having to enable/configure a Hop Link or Direct Link for that particular product; then affiliates would only be offered the one, common Hop Link to make things easy for them.
Clicking any Hop Link (whether common or product-specific) sets a cookie in that buyer's browser that expires in 6 months, so anything that buyer purchases from you during the lifetime of that cookie will earn the referring affiliate the appropriate percentage (i.e., earns them the product-specific percentage for products that have one, and otherwise earns them the common percentage for all other products purchased).