ESPIONAGE TECHNOLOGY upgrades are always useful. If you have a higher tech level than your target you will usually get a full report with a single probe. If the target has the same or higher tech level you will need to send more. You can always get a full spy report if you send enough probes* no matter how high the targets tech level, so upgrades are really a matter of convenience rather than necessity.
*The ingame description of this tech says that you need level 8 to get full reports. To the best of my recollection this is false, but it has been a long time since I started, so perhaps a new player can confirm if this is the case or not?
Espionage Reports have 5 components:
Raw Materials shows you the resources on the planet or moon. Probes will always show this. This part of the report will also include the planet name and co-ords.
Fleet shows you the ships in orbit including solar satellites.
Defenses shows you the planetary defensive units such as rocket launchers and plasma cannons.
Buildings shows you the buildings on the planet.
Research shows you the target's research levels.
These headings will always be revealed in the order in which they are listed above i.e. you will never get a Research report without also getting the other four, but you may get a Raw Materials and Fleet report without getting the rest.
IMPORTANT: If a heading appears then the probe was successful for that component. If there is nothing underneath the heading then there is nothing there. HOWEVER, if a heading does not appear in the report it means the probe FAILED in that regard. So if there is no Defenses heading, it does not mean there is no defense, it means the probe failed to determine the defenses. Send more probes.
COMPUTER TECHNOLOGY upgrades are always useful. You start with one fleet slot (meaning you can have one fleet mission active at one time) and each tech upgrade increases this by one.
Also, CT4 is a prerequisite for Expedition Technology, CT8 for Intergalactic Research Network, and CT10 for Nanite Factories.
WEAPONS/SHIELD/ARMOR TECHNOLOGY upgrades are always useful. Every ship and defense has a weapons value, shield value and armor value (wsa). Each level upgrade increases the respective value by 10% of the BASE value.
ENERGY TECHNOLOGY upgrades are always useful IF you use Fusion Reactors. Each level of tech increases the energy output of FRs.
ET1 is a prerequisite for Light Lasers, ET2 for Laser Technology, ET3 for Heavy Lasers, FRs and Shield Technology, ET4 for Ion Technology, ET5 for Hyperspace Technology, ET6 for Gauss Cannons, ET8 for Plasma Technology,and ET12 for Terraformers.
HYPERSPACE TECHNOLOGY upgrades are useful ONLY as a prerequisite for other things; HT3 for Hyperspace ENGINE, HT5 for Destroyers and Battlecruisers, HT6 for Death Stars, HT7 for Jump Gates, HT8 for Intergalactic Research Network. and HT9 for Lunar Guardians (available only in USA). Beyond level 8 in Univ 1 and level 9 in USA, it has no use. Also, be careful to distinguish Hyperspace TECHNOLOGY and Hyperspace ENGINE.
COMBUSTION ENGINE upgrades are always useful, each level increasing the speed of small cargos (SC), large cargos (LC), light fighters (LF), recyclers (recs) and espionage probes (EPs) by 10% of the BASE speed.
CE1 is a prerequisite for LFs, CE2 for SCs, CE3 for EPs, and CE6 for LCs and recs.
IMPULSE ENGINE upgrades are always useful, each level increasing the speed of bombers (BB), cruisers, heavy fighters (HF), colony ships and Elite Cargos (EC) by 20% of the BASE speed. EC are only available in USA.
IE2 is a prerequisite for HFs, IE3 for Colony Ships, IE4 for Cruisers
Also, your IE level determines the range of your Inter-Planetary Missiles (IPMs), which is 5 per IE level.
HYPERSPACE ENGINE upgrades are always useful, each level increasing the speed of Battleships (BS), Battlecruisers (BC), Destroyers (DD), Death Stars (RIPs) and Lunar Guardians (LG) by 30% of the BASE speed. LG are only available in USA.
LASER TECHNOLOGY upgrades are useful ONLY as prerequisites for other things: LT3 is a prerequisite for Light Lasers, LT5 for Ion Technology, LT6 for Heavy Lasers, LT10 for Plasma Technology, and LT12 for Battlecruisers. After that, upgrades are useless.
ION TECHNOLOGY upgrades are useful ONLY as prerequisites for other things: IT2 is a prerequisite for Cruisers, IT4 for Ion Cannons, and IT5 for Plasma Technology. After that, upgrades are useless.
PLASMA TECHNOLOGY upgrades are always useful IN USA ONLY. IN USA ONLY each upgrade increases the Weapons value of Plasma Cannons and Bombers by 5% of the BASE value. In Univ 1 it does not do this.
PT5 is a prerequisite for Bombers and PT7 for Plasma Cannons. In Univ 1 upgrades are useless after that.
INTERGALACTIC RESEARCH NETWORK upgrades are always useful if you have at least as many Research Labs as the level of your IRN. With IRN level n you will always research at the combined level of your n highest Labs regardless of which Lab in which you begin the research (but the chosen Lab must still itself have the requisite level for the research). So, at level 1 you will always research at the speed of your highest Lab regardless of which lab in which you start the research, at level 2 you will research at the speed of your two highest Lab levels combined, etc.
EXPEDITION TECHNOLOGY upgrades allow you to send an additional Expedition every few levels. There are several possible outcomes of Expeditions: you find nothing, you find some resources (but it wont be much), you find ships, you find some nebulae and take some nice pictures or some such, and you can find a black hole and lose some of the Expedition fleet to it.
SERIOS PHYSICS is available only in USA. Upgrades are useful ONLY as prerequisites for the Elite Cargo and Lunar Guardian; SP2 for ECs and SP4 for LGs. After that upgrades are useless.
GRAVITON TECHNOLOGY upgrades are useful ONLY as prerequisites for the Death Star (RIP) and Lunar Guardian (LG); GT1 for RIPs, GT2 for LGs. LGs are available in USA ONLY. After that upgrades are useless. Also, once you have the requisite energy, Graviton Tech researches instantly when you click on it.