On putting off marriage, the Rebbe's guidance is consistent: don't delay without real reason.
Financial and housing concerns are not a reason to postpone. Delaying marriage over money or social considerations has no real basis; trust in Hashem's providence and look for practical solutions, such as help from friends or a more affordable Torah community, rather than putting off the wedding (IK 5892, 6305, 8743). In a shidduch, personal qualities come before financial considerations (IK 1578). The Rebbe also called on communities to find collective solutions to high housing costs that delay marriages (IK 4736).
Continued learning is not a reason to delay either, when arrangements can be made to keep learning after the wedding (IK 5369). Delaying unnecessarily is discouraged, and a relationship should move forward only when one is ready for marriage (IK 7512).
At the same time, don't pressure young people. A bochur focused on his learning should not be rushed into shidduchim or worries about the future; trust in Hashem's timing (IK 2700, 9367).
On the time between engagement and the wedding, see: שידוכים – זמן בין אירוסין לחתונה.