表示未能实现的过去计划、意图、打算等时，有二种结构可选用（以plan为例）：

I had planned to buy a car. 或者

I planned to have bought a car.

以上二句的意思是：我曾计划买一辆车，但这个计划未能实现。翻译为：我本打算买辆车的。与下句比较：

I planned to buy a car. （当时）我计划买一辆车。（车后来买了没有不得而知，可能后来买了，也可能后来没有买。说话人只是告诉听话人当初有这个计划，仅此而已。）

你提问的二个句子与此类似。表示的意思是说话人过去错误的想象或假设的情况事实证明是错误的。所以谓语用了过去完成时，后接不定式。也可以用一般过去时后接不定式完成体。

I had imagined her to be older than that.

= I imagined her to have been older than that.

She had supposed him (to be) very rich.

= She supposed him to have been very rich.

注意汉译中的“原以为”、“本以为”，表示这种以为已经证明是错的。

比较：

I imagined her to be older than that.

She supposed him (to be) very rich.

这二个句子仅仅表示主语当时有这种看法，是否正确不得而知。