可以肯定的是，ideal for first-time buyers 不是作表语，因为从结构上看small and inexpensive 才是并列成分，它们之间有and 连接。那么，ideal for first-time buyers 实际上是个非限制性定语，相当于一个非限制性定语从句。

The houses are small and inexpensive, ideal for first-time buyers.  相当于：

The houses are small and inexpensive, which is ideal for first-time buyers. 

which 指代整个主句的内容，即：“这些房子既小又便宜”这个事实。

整个句子意思是：

“这些房子小而便宜，这是首次购房者的理想选择。”