If we say "Get a mortgage loan with as little as X % down and rates as low as X.XX% APR" are there any Reg Z or UDAAP concerns? The ad will include a disclosure for a sample loan to include the terms of repayment.

As long as you have a downpayment, you need to have the rest of the disclosures on 1026.24.

'(d) Advertisement of terms that require additional disclosures. (1) Triggering terms. If any of the following terms is set forth in an advertisement, the advertisement shall meet the requirements of paragraph (d)(2) of this section:
(i) The amount or percentage of any downpayment.
(ii) The number of payments or period of repayment.
(iii) The amount of any payment.
(iv) The amount of any finance charge.

(2) Additional terms. An advertisement stating any of the terms in paragraph (d)(1) of this section shall state the following terms, as applicable (an example of one or more typical extensions of credit with a statement of all the terms applicable to each may be used):
(i) The amount or percentage of the downpayment.
(ii) The terms of repayment, which reflect the repayment obligations over the full term of the loan, including any balloon payment.
(iii) The “annual percentage rate,” using that term, and, if the rate may be increased after consummation, that fact.

Downpayment is only a triggering term if this is credit sale - as the creditor is the seller of the goods.

24(d)(1) Triggering Terms
1. Downpayment. i. The dollar amount of a downpayment or a statement of the downpayment as a percentage of the price requires further information. By virtue of the definition of downpayment in §1026.2, this triggering term is limited to credit sale transactions.

The fact that there is a downpayment in a real estate secured transaction is not the determining factor. What matters is the identity of the "person" receiving the payment. That person is always the seller of the property (real or personal.) "Downpayment" is relevant to Reg. Z disclosures ONLY when the bank is both the seller of the property AND the creditor. The most common case where this happens is self-financed sales of foreclosed real property. The same principle would apply to self-financed sales of repossessed personal property -- but in the real world, that would be a rare transaction. The only other "covered" (but rare) case I can imagine is a self-financed sale of OREO.