The products offer period is from today to 31 May 2019.
Cashback of £1,250 will be offered on loans between £250,001 and £1m, and on loans less than or equal to £250,000 there will be a cashback of £750 at completion.
The purchase only exclusive range provides products at 65% and 75% loan to value (LTV) with rates starting from 2.99% for applications on standard buy to let property.
Rates start from 3.19% for purchase applications on houses in multiple occupation (HMOs) and multi-unit blocks.
CEO David Whittaker said: “As our first addition to our product range since successfully relaunching into the buy to let market in September 2018 we are delighted to be able to offer some new and exciting purchase only products.
He added that landlords have seen a number of changes over the past two and a half years but they “are very resilient and have a healthy appetite to look at further buy to let purchases”.
Keystone’s products are now available to members of The Right Mortgage and Protection Network via Legal and General Mortgage Club, and Lighthouse.
Martin Wilson, CEO at The Right Mortgage and Protection Network, said: “We hope our members will be able to further assist their landlord clients who have complex borrowing requirements, thanks to Keystone’s product range.
“Combined with their cloud based application system we are confident that brokers will be able transact cases quickly and efficiently.”
Paul Harris, managing director of Lighthouse Mortgage and Protection Solutions, added: “With the buy-to-let market becoming more complex, we are pleased to be able to add another specialist lender to our panel.
“Keystone provides a wide range of criteria and solutions for complex buy-to-let cases which we hope our advisers will benefit from.”