Guess conveys tentativeness. What would you think of a CEO who tells analysts:

• Our best guess is that our profit for year’s end will be marginally better than last year’s.

This is weak and makes the CEO sound completely disconnected from the business. It can be reframed more confidently as:

• We expect our profit for the year to be ahead of last year’s.
• Our results should surpass last year’s but this hasn’t been confirmed by the CFO.

The following can be rephrased:

• I guess our debt will be reduced by year’s end. [weak]
  
• Our debt will be reduced by year’s end. [strong]
  

Be careful with get: it can be vague; choose a more accurate verb:  

• We require more funds to complete the project.:  

Informally, get is frequently used but, when writing, make sure the verb balances with the true meaning as get can weaken your comment or request:  

• Can we get these products to the customer by tomorrow? [weak]
• Can we send these products to the customer by tomorrow? [stronger]
• Can we courier these products to the customer by tomorrow? [expresses urgency]

In the first sentence below, get could refer to buy, enquire, investigate or something else:

• We need to get a suitable IT system. [weak]
  
• We need to invest in a suitable IT system. [strong]
  

Be careful with guess: it can convey tentativeness, lack of confidence or incompetence. Informally, it is used to indicate that although one thinks or supposes something, it is without any great conviction or strength of feeling.

If you are unsure, say so and follow with how you will solve the situation:

• These are the preliminary figures. The complete numbers will be with you by the end of the week.
  
• I will ring the client tomorrow morning and check they fully understand the figures.